Gui: Add ThemeTokenManager class to contain theme parameters

This class aims to implement Design Token idea into FreeCAD themes. It
allows themes to use generic variables with generic values so we could
use one qss theme and change the style based on values from preference
packs.

src/Gui/PreferencePackManager.cpp

@@ -41,9 +41,12 @@
 #include "DockWindowManager.h"
 #include "ToolBarManager.h"
 
+#include <Application.h>
 #include <App/Application.h>
 
-#include <ctime> // For generating a timestamped filename
+#include <ctime>  // For generating a timestamped filename
+#include <Base/ServiceProvider.h>
+#include <Dialogs/DlgThemeEditor.h>
 
 
 using namespace Gui;
@@ -359,6 +362,9 @@ bool PreferencePackManager::apply(const std::string& preferencePackName) const
             Gui::ToolBarManager* pToolbarMgr = Gui::ToolBarManager::getInstance();
             pToolbarMgr->restoreState();
 
+            // We need to reload stylesheet to apply any changed style parameters
+            Gui::Application::Instance->reloadStyleSheet();
+
             // TODO: Are there other things that have to be manually triggered?
         }
         return wasApplied;