v3 of the site doesn't allow creation of themes. You can upload a background and you can tweaks some settings: https://www.chess.com/forum/view/community/chess-com-s-new-open-source-browser-extension (currently Chrome only).
There are also some 3d style pieces.
Where 1000 3d themes and 2000 3d pieces.
Users can make own theme and upload on chess.com for others