After applying the desktop background ... In the RetroBar Properties window, click the dropdown icon next to Theme and select "Windows XP Blue.". Additionally, check the boxes for "Allow Font ...

In addition to Windows XP, you can also ... any time using the ‘XPQ4 Desktop Styles’ tool. The different Windows versions are available for selection in the ‘Themes’ column.

With those Windows tweaks taken care of, download the Classic Shell XP suite from Winaero ... right-click on a blank area of the desktop and choose Personalize. Choose Themes then Theme settings ...