GIMP doesn't have its own built-in themes, but you can download some from other websites like GimpStuff. Once you get a theme, it needs to be placed in the following directory C:\Program Files\GIMP 2\share\gimp\2.0\themes. Once the files have been placed in the directory above, restart GIMP, then go to Edit > Preferences and click the Themes section on the left, then you can change them.
