Add theme management and background rendering features

- Introduced a comprehensive theme management system, allowing users to load, save, and switch between multiple themes.
- Added support for various built-in themes, enhancing the visual customization of the application.
- Implemented a background renderer for improved visual effects in docking windows, including grid backgrounds and subtle animations.
- Enhanced the EditorManager UI with themed elements, providing a more cohesive and engaging user experience.
- Updated CMake configuration to include new theme and background renderer source files, ensuring proper integration into the build system.

src/app/core/window.cc

@@ -6,7 +6,9 @@
 #include "app/core/platform/sdl_deleter.h"
 #include "app/gfx/arena.h"
 #include "app/gui/style.h"
+#include "app/gui/theme_manager.h"
 #include "app/test/test_manager.h"
+#include "util/log.h"
 #include "imgui/backends/imgui_impl_sdl2.h"
 #include "imgui/backends/imgui_impl_sdlrenderer2.h"
 #include "imgui/imgui.h"
@@ -49,7 +51,16 @@ absl::Status CreateWindow(Window& window, int flags) {
 
   RETURN_IF_ERROR(LoadPackageFonts());
 
+  // Apply original YAZE colors as fallback, then try to load theme system
   gui::ColorsYaze();
+  
+  // Try to initialize theme system (will fallback to ColorsYaze if files fail)
+  auto& theme_manager = gui::ThemeManager::Get();
+  auto status = theme_manager.LoadTheme("YAZE Classic");
+  if (!status.ok()) {
+    // Theme system failed, stick with original ColorsYaze()
+    util::logf("Theme system failed, using original ColorsYaze(): %s", status.message().data());
+  }
 
   const int audio_frequency = 48000;
   SDL_AudioSpec want, have;