UI: Ensure theme backward compat. with older OBS vers.

Deprecates "Theme" global config value and uses "CurrentTheme" instead
to ensure backward compatibility with older versions of OBS, which will
fail to load if you have a theme that isn't available.

(Jim Edit: Putting this back because I misunderstood the purpose of
jp9000/obs-studio#936)

UI/window-basic-settings.cpp

@@ -2436,8 +2436,9 @@ void OBSBasicSettings::SaveGeneralSettings()
 	string theme = themeData.toStdString();
 
 	if (WidgetChanged(ui->theme)) {
-		config_set_string(GetGlobalConfig(), "General", "Theme",
+		config_set_string(GetGlobalConfig(), "General", "CurrentTheme",
 				  theme.c_str());
+
 		App()->SetTheme(theme);
 	}