feat: add system theme on Qt 6.5 and up (#5118)

* feat: add system theme on Qt 6.5

* chroe: add changelog entry

* refactor: add separate settings

* fix: qt 5

* Update changelog entry

---------

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/singletons/Theme.hpp

@@ -46,6 +46,7 @@ public:
     void initialize(Settings &settings, const Paths &paths) final;
 
     bool isLightTheme() const;
+    bool isSystemTheme() const;
 
     struct TabColors {
         QColor text;
@@ -153,6 +154,9 @@ public:
     pajlada::Signals::NoArgSignal updated;
 
     QStringSetting themeName{"/appearance/theme/name", "Dark"};
+    QStringSetting lightSystemThemeName{"/appearance/theme/lightSystem",
+                                        "Light"};
+    QStringSetting darkSystemThemeName{"/appearance/theme/darkSystem", "Dark"};
 
 private:
     bool isLight_ = false;
@@ -164,6 +168,8 @@ private:
     // This will only be populated when auto-reloading themes
     QJsonObject currentThemeJson_;
 
+    QObject lifetime_;
+
     /**
      * Figure out which themes are available in the Themes directory
      *