diff --git a/src/Wt/WBootstrap2Theme.C b/src/Wt/WBootstrap2Theme.C
index 3c9c6411..226b6649 100644
--- a/src/Wt/WBootstrap2Theme.C
+++ b/src/Wt/WBootstrap2Theme.C
@@ -10,6 +10,7 @@
 #include "Wt/WAbstractSpinBox.h"
 #include "Wt/WApplication.h"
 #include "Wt/WCheckBox.h"
+#include "Wt/WCompositeWidget.h"
 #include "Wt/WDateEdit.h"
 #include "Wt/WDatePicker.h"
 #include "Wt/WDialog.h"
@@ -204,6 +205,8 @@ void WBootstrap2Theme::apply(WWidget *widget,
   if (!widget->isThemeStyleEnabled())
     return;
 
+  Wt::WCompositeWidget *composite = dynamic_cast<WCompositeWidget*>(widget);
+
   {
     WPopupWidget *popup = dynamic_cast<WPopupWidget *>(widget);
     if (popup) {
@@ -265,19 +268,19 @@ void WBootstrap2Theme::apply(WWidget *widget,
   case DomElementType::DIV:
     {
       WDialog *dialog = dynamic_cast<WDialog *>(widget);
-      if (dialog) {
+      if (dialog || (composite && composite->canCastTo<WDialog*>())) {
         element.addPropertyWord(Property::Class, "modal");
         return;
       }
 
       WPanel *panel = dynamic_cast<WPanel *>(widget);
-      if (panel) {
+      if (panel || (composite && composite->canCastTo<WPanel*>())) {
         element.addPropertyWord(Property::Class, "accordion-group");
         return;
       }
 
       WProgressBar *bar = dynamic_cast<WProgressBar *>(widget);
-      if (bar) {
+      if (bar || (composite && composite->canCastTo<WProgressBar*>())) {
         switch (elementRole) {
         case MainElement:
           element.addPropertyWord(Property::Class, "progress");
@@ -293,19 +296,19 @@ void WBootstrap2Theme::apply(WWidget *widget,
       }
 
       WGoogleMap *map = dynamic_cast<WGoogleMap *>(widget);
-      if (map) {
+      if (map || (composite && composite->canCastTo<WGoogleMap*>())) {
         element.addPropertyWord(Property::Class, "Wt-googlemap");
         return;
       }
 
       WAbstractItemView *itemView = dynamic_cast<WAbstractItemView *>(widget);
-      if (itemView) {
+      if (itemView || (composite && composite->canCastTo<WAbstractItemView*>())) {
         element.addPropertyWord(Property::Class, "form-inline");
         return;
       }
 
       WNavigationBar *navBar = dynamic_cast<WNavigationBar *>(widget);
-      if (navBar) {
+      if (navBar || (composite && composite->canCastTo<WNavigationBar*>())) {
         element.addPropertyWord(Property::Class, "navbar");
         return;
       }
@@ -356,19 +359,19 @@ void WBootstrap2Theme::apply(WWidget *widget,
   case DomElementType::INPUT:
     {
       WAbstractSpinBox *spinBox = dynamic_cast<WAbstractSpinBox *>(widget);
-      if (spinBox) {
+      if (spinBox || (composite && composite->canCastTo<WAbstractSpinBox*>())) {
         element.addPropertyWord(Property::Class, "Wt-spinbox");
         return;
       }
 
       WDateEdit *dateEdit = dynamic_cast<WDateEdit *>(widget);
-      if (dateEdit) {
+      if (dateEdit || (composite && composite->canCastTo<WDateEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-dateedit");
         return;
       }
 
       WTimeEdit *timeEdit = dynamic_cast<WTimeEdit *>(widget);
-      if (timeEdit) {
+      if (timeEdit || (composite && composite->canCastTo<WTimeEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-timeedit");
         return;
       }
@@ -410,12 +413,12 @@ void WBootstrap2Theme::apply(WWidget *widget,
     {
       WInPlaceEdit *inPlaceEdit
         = dynamic_cast<WInPlaceEdit *>(widget);
-      if (inPlaceEdit)
+      if (inPlaceEdit || (composite && composite->canCastTo<WInPlaceEdit*>()))
         element.addPropertyWord(Property::Class, "Wt-in-place-edit");
       else {
         WDatePicker *picker
           = dynamic_cast<WDatePicker *>(widget);
-        if (picker)
+        if (picker || (composite && composite->canCastTo<WDatePicker*>()))
           element.addPropertyWord(Property::Class, "Wt-datepicker");
       }
 
diff --git a/src/Wt/WBootstrap3Theme.C b/src/Wt/WBootstrap3Theme.C
index dc3d6228..9614ec66 100644
--- a/src/Wt/WBootstrap3Theme.C
+++ b/src/Wt/WBootstrap3Theme.C
@@ -12,6 +12,7 @@
 #include "Wt/WBootstrapTheme.h"
 #include "Wt/WCheckBox.h"
 #include "Wt/WComboBox.h"
+#include "Wt/WCompositeWidget.h"
 #include "Wt/WDateEdit.h"
 #include "Wt/WDatePicker.h"
 #include "Wt/WLinkedCssStyleSheet.h"
@@ -236,6 +237,8 @@ void WBootstrap3Theme::apply(WWidget *widget, DomElement& element,
   if (!widget->isThemeStyleEnabled())
     return;
 
+  Wt::WCompositeWidget *composite = dynamic_cast<WCompositeWidget*>(widget);
+  
   {
     WPopupWidget *popup = dynamic_cast<WPopupWidget *>(widget);
     if (popup) {
@@ -297,19 +300,19 @@ void WBootstrap3Theme::apply(WWidget *widget, DomElement& element,
   case DomElementType::DIV:
     {
       WDialog *dialog = dynamic_cast<WDialog *>(widget);
-      if (dialog) {
+      if (dialog || (composite && composite->canCastTo<WDialog*>())) {
         element.addPropertyWord(Property::Class, "modal-dialog Wt-dialog");
         return;
       }
 
       WPanel *panel = dynamic_cast<WPanel *>(widget);
-      if (panel) {
+      if (panel || (composite && composite->canCastTo<WPanel*>())) {
         element.addPropertyWord(Property::Class, "panel panel-default");
         return;
       }
 
       WProgressBar *bar = dynamic_cast<WProgressBar *>(widget);
-      if (bar) {
+      if (bar || (composite && composite->canCastTo<WProgressBar*>())) {
         switch (elementRole) {
         case MainElement:
           element.addPropertyWord(Property::Class, "progress");
@@ -325,19 +328,19 @@ void WBootstrap3Theme::apply(WWidget *widget, DomElement& element,
       }
 
       WGoogleMap *map = dynamic_cast<WGoogleMap *>(widget);
-      if (map) {
+      if (map || (composite && composite->canCastTo<WGoogleMap*>())) {
         element.addPropertyWord(Property::Class, "Wt-googlemap");
         return;
       }
 
       WAbstractItemView *itemView = dynamic_cast<WAbstractItemView *>(widget);
-      if (itemView) {
+      if (itemView || (composite && composite->canCastTo<WAbstractItemView*>())) {
         element.addPropertyWord(Property::Class, "form-inline");
         return;
       }
 
       WNavigationBar *navBar = dynamic_cast<WNavigationBar *>(widget);
-      if (navBar) {
+      if (navBar || (composite && composite->canCastTo<WNavigationBar*>())) {
         element.addPropertyWord(Property::Class, "navbar navbar-default");
         return;
       }
@@ -394,19 +397,19 @@ void WBootstrap3Theme::apply(WWidget *widget, DomElement& element,
       }
 
       WAbstractSpinBox *spinBox = dynamic_cast<WAbstractSpinBox *>(widget);
-      if (spinBox) {
+      if (spinBox || (composite && composite->canCastTo<WAbstractSpinBox*>())) {
         element.addPropertyWord(Property::Class, "Wt-spinbox");
         return;
       }
 
       WDateEdit *dateEdit = dynamic_cast<WDateEdit *>(widget);
-      if (dateEdit) {
+      if (dateEdit || (composite && composite->canCastTo<WDateEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-dateedit");
         return;
       }
 
       WTimeEdit *timeEdit = dynamic_cast<WTimeEdit *>(widget);
-      if (timeEdit) {
+      if (timeEdit || (composite && composite->canCastTo<WTimeEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-timeedit");
         return;
       }
@@ -454,12 +457,12 @@ void WBootstrap3Theme::apply(WWidget *widget, DomElement& element,
     {
       WInPlaceEdit *inPlaceEdit
         = dynamic_cast<WInPlaceEdit *>(widget);
-      if (inPlaceEdit)
+      if (inPlaceEdit || (composite && composite->canCastTo<WInPlaceEdit*>()))
         element.addPropertyWord(Property::Class, "Wt-in-place-edit");
       else {
         WDatePicker *picker
           = dynamic_cast<WDatePicker *>(widget);
-        if (picker)
+        if (picker || (composite && composite->canCastTo<WDatePicker*>()))
           element.addPropertyWord(Property::Class, "Wt-datepicker");
       }
     }
diff --git a/src/Wt/WBootstrap5Theme.C b/src/Wt/WBootstrap5Theme.C
index 6ac05fa2..f390eff5 100644
--- a/src/Wt/WBootstrap5Theme.C
+++ b/src/Wt/WBootstrap5Theme.C
@@ -10,6 +10,7 @@
 #include "Wt/WAbstractSpinBox.h"
 #include "Wt/WApplication.h"
 #include "Wt/WCheckBox.h"
+#include "Wt/WCompositeWidget.h"
 #include "Wt/WDateEdit.h"
 #include "Wt/WDatePicker.h"
 #include "Wt/WLinkedCssStyleSheet.h"
@@ -245,6 +246,9 @@ void WBootstrap5Theme::apply(WWidget *widget, DomElement& element,
   if (!widget->isThemeStyleEnabled())
     return;
 
+  
+  Wt::WCompositeWidget *composite = dynamic_cast<WCompositeWidget*>(widget);
+
   {
     auto popup = dynamic_cast<WPopupWidget *>(widget);
     if (popup) {
@@ -306,19 +310,19 @@ void WBootstrap5Theme::apply(WWidget *widget, DomElement& element,
 
   case DomElementType::DIV: {
     auto dialog = dynamic_cast<WDialog *>(widget);
-    if (dialog) {
+    if (dialog || (composite && composite->canCastTo<WDialog*>())) {
       element.addPropertyWord(Property::Class, "modal Wt-dialog");
       return;
     }
 
     auto panel = dynamic_cast<WPanel *>(widget);
-    if (panel) {
+    if (panel || (composite && composite->canCastTo<WPanel*>())) {
       element.addPropertyWord(Property::Class, "card Wt-panel");
       return;
     }
 
     auto bar = dynamic_cast<WProgressBar *>(widget);
-    if (bar) {
+    if (bar || (composite && composite->canCastTo<WProgressBar*>())) {
       switch (elementRole) {
       case MainElement:
         element.addPropertyWord(Property::Class, "progress");
@@ -331,7 +335,7 @@ void WBootstrap5Theme::apply(WWidget *widget, DomElement& element,
     }
 
     auto map = dynamic_cast<WGoogleMap *>(widget);
-    if (map) {
+    if (map || (composite && composite->canCastTo<WGoogleMap*>())) {
       element.addPropertyWord(Property::Class, "Wt-googlemap");
       return;
     }
@@ -415,19 +419,19 @@ void WBootstrap5Theme::apply(WWidget *widget, DomElement& element,
     }
 
     auto spinBox = dynamic_cast<WAbstractSpinBox *>(widget);
-    if (spinBox) {
+    if (spinBox || (composite && composite->canCastTo<WAbstractSpinBox*>())) {
       element.addPropertyWord(Property::Class, "Wt-spinbox");
       return;
     }
 
     auto dateEdit = dynamic_cast<WDateEdit *>(widget);
-    if (dateEdit) {
+    if (dateEdit || (composite && composite->canCastTo<WDateEdit*>())) {
       element.addPropertyWord(Property::Class, "Wt-dateedit");
       return;
     }
 
     auto timeEdit = dynamic_cast<WTimeEdit *>(widget);
-    if (timeEdit) {
+    if (timeEdit || (composite && composite->canCastTo<WTimeEdit*>())) {
       element.addPropertyWord(Property::Class, "Wt-timeedit");
       return;
     }
@@ -481,11 +485,11 @@ void WBootstrap5Theme::apply(WWidget *widget, DomElement& element,
       element.addPropertyWord(Property::Class, "form-file-button");
 
     auto inPlaceEdit = dynamic_cast<WInPlaceEdit *>(widget);
-    if (inPlaceEdit)
+    if (inPlaceEdit || (composite && composite->canCastTo<WInPlaceEdit*>()))
       element.addPropertyWord(Property::Class, "Wt-in-place-edit");
     else {
       auto picker = dynamic_cast<WDatePicker *>(widget);
-      if (picker)
+      if (picker || (composite && composite->canCastTo<WDatePicker*>()))
         element.addPropertyWord(Property::Class, "Wt-datepicker");
     }
   }
diff --git a/src/Wt/WCompositeWidget.h b/src/Wt/WCompositeWidget.h
index bb3367b9..1da8000e 100644
--- a/src/Wt/WCompositeWidget.h
+++ b/src/Wt/WCompositeWidget.h
@@ -9,6 +9,11 @@
 
 #include <Wt/WWidget.h>
 
+// Conditionally include <concepts> ONLY IF the compiler supports C++20 Concepts
+#ifdef __cpp_concepts
+#include <concepts>
+#endif
+
 namespace Wt {
 
 /*! \class WCompositeWidget Wt/WCompositeWidget.h Wt/WCompositeWidget.h
@@ -63,6 +68,36 @@ public:
 
   ~WCompositeWidget();
 
+  /**
+   * @brief helper Function for Theme::apply to check if underlying impl can be cast to predefined Widgets
+   * \warning Not Every Widget is Supported!
+   */
+  template <typename T>
+    #ifdef __cpp_concepts
+      // Constraint 1: T must be a pointer type
+      // Constraint 2: The type T points to (std::remove_pointer_t<T>) must be WWidget or derived from it.
+      requires std::is_pointer_v<T> && (std::derived_from<std::remove_pointer_t<T>, WWidget> || std::same_as<std::remove_pointer_t<T>, WWidget>)
+    #endif
+    bool canCastTo() const {
+        // Fallback static_assert for older compilers
+      #ifndef __cpp_concepts
+        // Ensure T is a pointer type
+        static_assert(std::is_pointer<T>::value, 
+                      "Template argument T must be a pointer type (e.g., WPanel*).");
+
+        // Ensure the pointed-to type is derived from WWidget
+        static_assert(std::is_base_of<WWidget, std::remove_pointer_t<T>>::value, 
+                      "The type pointed to by T must be a WWidget or derived from WWidget.");
+      #endif
+
+      if (!impl_) {
+          return false; // Cannot cast if no implementation widget exists
+      }
+      
+      // Perform the dynamic_cast and check if it succeeds
+      return dynamic_cast<T>(impl_.get()) != nullptr; 
+  }
+
   virtual std::vector<WWidget *> children() const override;
   using WWidget::removeWidget;
   virtual std::unique_ptr<WWidget> removeWidget(WWidget *widget) override;
diff --git a/src/Wt/WCssTheme.C b/src/Wt/WCssTheme.C
index e1db08fb..366eebba 100644
--- a/src/Wt/WCssTheme.C
+++ b/src/Wt/WCssTheme.C
@@ -11,6 +11,7 @@
 #include "Wt/WApplication.h"
 #include "Wt/WCalendar.h"
 #include "Wt/WCssTheme.h"
+#include "Wt/WCompositeWidget.h"
 #include "Wt/WDateEdit.h"
 #include "Wt/WDialog.h"
 #include "Wt/WEnvironment.h"
@@ -162,6 +163,8 @@ void WCssTheme::apply(WWidget *widget, DomElement& element, int elementRole)
       element.addPropertyWord(Property::Class, "Wt-outset");
   }
 
+  Wt::WCompositeWidget *composite = dynamic_cast<WCompositeWidget*>(widget);
+
   switch (element.type()) {
   case DomElementType::BUTTON:
     if (creating) {
@@ -213,19 +216,19 @@ void WCssTheme::apply(WWidget *widget, DomElement& element, int elementRole)
   case DomElementType::DIV:
     {
       WDialog *dialog = dynamic_cast<WDialog *>(widget);
-      if (dialog) {
+      if (dialog || (composite && composite->canCastTo<WDialog*>())) {
         element.addPropertyWord(Property::Class, "Wt-dialog");
         return;
       }
 
       WPanel *panel = dynamic_cast<WPanel *>(widget);
-      if (panel) {
+      if (panel || (composite && composite->canCastTo<WPanel*>())) {
         element.addPropertyWord(Property::Class, "Wt-panel Wt-outset");
         return;
       }
 
       WProgressBar *bar = dynamic_cast<WProgressBar *>(widget);
-      if (bar) {
+      if (bar || (composite && composite->canCastTo<WProgressBar*>())) {
         switch (elementRole) {
         case MainElement:
           element.addPropertyWord(Property::Class, "Wt-progressbar");
@@ -245,19 +248,19 @@ void WCssTheme::apply(WWidget *widget, DomElement& element, int elementRole)
   case DomElementType::INPUT:
     {
       WAbstractSpinBox *spinBox = dynamic_cast<WAbstractSpinBox *>(widget);
-      if (spinBox) {
+      if (spinBox || (composite && composite->canCastTo<WAbstractSpinBox*>())) {
         element.addPropertyWord(Property::Class, "Wt-spinbox");
         return;
       }
 
       WDateEdit *dateEdit = dynamic_cast<WDateEdit *>(widget);
-      if (dateEdit) {
+      if (dateEdit || (composite && composite->canCastTo<WDateEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-dateedit");
         return;
       }
 
       WTimeEdit *timeEdit = dynamic_cast<WTimeEdit *>(widget);
-      if (timeEdit) {
+      if (timeEdit || (composite && composite->canCastTo<WTimeEdit*>())) {
         element.addPropertyWord(Property::Class, "Wt-timeedit");
         return;
       }