Fix compare dialog alternate background color on windows (#10781)

**PROBLEM**
• Row colors not updating after switching dark mode and opening dialog again
• Fixes https://github.com/SoftFever/OrcaSlicer/issues/9902 and https://github.com/SoftFever/OrcaSlicer/issues/966

**IMPORTANT NOTE**
• Main source of problem: dialog not destroyed after closing

**BEFORE**
<img width="769" height="682" alt="Screenshot-20250918134833" src="https://github.com/user-attachments/assets/dd77696c-ad2d-4de7-aaab-f85dd558dbbe" />

**AFTER**
<img width="757" height="677" alt="Screenshot-20250918134857" src="https://github.com/user-attachments/assets/fd28941d-996d-410f-ab04-b13f1dd0170f" />

src/slic3r/GUI/MainFrame.cpp

@@ -2271,6 +2271,8 @@ void MainFrame::on_sys_color_changed()
 #endif
 #endif
 
+    diff_dialog.on_sys_color_changed();
+
     // BBS
     m_tabpanel->Rescale();
     m_param_panel->msw_rescale();

src/slic3r/GUI/UnsavedChangesDialog.hpp

@@ -463,7 +463,6 @@ public:
 
     std::array<Preset::Type, 3>         types_list() const;
 
-protected:
     void on_dpi_changed(const wxRect& suggested_rect) override;
     void on_sys_color_changed() override;
 };