ui: Set Link Color when setting theme

Long story short, QT doesn't allow the link colors to be set via their stylesheets.

There are two ways to work with this, specify the color manually for every link (See the About dialog) The other way is to change the default palette.

IsDarkTheme is copy/pasted from src/yuzu/debugger/wait_tree.cpp

src/yuzu/uisettings.cpp

@@ -15,6 +15,14 @@ const Themes themes{{
     {"Midnight Blue Colorful", "colorful_midnight_blue"},
 }};
 
+bool IsDarkTheme() {
+    const auto& theme = UISettings::values.theme;
+    return theme == QStringLiteral("qdarkstyle") ||
+           theme == QStringLiteral("qdarkstyle_midnight_blue") ||
+           theme == QStringLiteral("colorful_dark") ||
+           theme == QStringLiteral("colorful_midnight_blue");
+}
+
 Values values = {};
 
 } // namespace UISettings