It's possible to edit the theme in Windows Style Builder yeah. Each theme comes with Aero and Basic images. What you can do is use the images from the Aero section in the theme for the Basic ones(so you won't have to create new ones for yourself).
You'll need to edit the .msstyles the theme came with.

Here are tutorials for the locations:
https://virtualcustoms.net/showthread...Theme-Building
https://virtualcustoms.net/showthread...askbar-Editing
https://virtualcustoms.net/showthread...Start-menu-etc

Keep in mind that Basic mode doesn't support semi transparent images.