[Qt] Implement theme selection

include/panda_qt/main_window.hpp

@@ -1,9 +1,9 @@
 #pragma once
 
 #include <QApplication>
+#include <QComboBox>
 #include <QMenuBar>
 #include <QPalette>
-#include <QPushBUtton>
 #include <QtWidgets>
 
 #include "panda_qt/screen.hpp"
@@ -12,11 +12,16 @@ class MainWindow : public QMainWindow {
 	Q_OBJECT
 
   private:
+	enum class Theme : int {
+		System = 0, Light = 1, Dark = 2,
+	};
+
+	QComboBox* themeSelect = nullptr;
 	QMenuBar* menuBar = nullptr;
-	QPushButton* themeButton = nullptr;
 	ScreenWidget screen;
 
-	void setDarkTheme();
+	Theme currentTheme;
+	void setTheme(Theme theme);
 
   public:
 	MainWindow(QApplication* app, QWidget* parent = nullptr);