FL Studio 21 introduced Theming ( Skins ). You can download it here. This is a free update for existing customers.
HISTORY
FL Studio 12 and 20 use a vectoral GUI (Graphical User Interface). It is not possible to modify the GUI without unauthorized changes to the FL Studio runtime code.
FL Studio 11 and older versions use bitmap images for the GUI. Where skins modify any part of the existing GUI bitmaps, this is an unauthorised use of our artwork.
LEGAL NOTICE:
Modification of the FL Studio GUI, outside the parameters provided in the program options, is a violation of Image-Line's copyright and moral rights. We expressly forbid anyone to alter, transform or build upon FL Studio logos or GUI artwork.
Image-Line Support Team