UI: Set default theme to Yami

Note that this adds a new CurrentTheme3 config variable to preserve
settings of versions older than 28, making this change safely
backward-compatible.

UI/window-basic-settings.cpp

@@ -3055,7 +3055,7 @@ void OBSBasicSettings::SaveGeneralSettings()
 	QString themeData = ui->theme->itemData(themeIndex).toString();
 
 	if (WidgetChanged(ui->theme))
-		config_set_string(GetGlobalConfig(), "General", "CurrentTheme2",
+		config_set_string(GetGlobalConfig(), "General", "CurrentTheme3",
 				  QT_TO_UTF8(themeData));
 
 #if defined(_WIN32) || defined(__APPLE__)