The Bitcoin UI Kit now has its own plugin to easily change the theme and keep the design system tidy. Here's a walkthrough.
https://www.youtube.com/watch?v=jiNFsR9aGlw
You can get it here on Figma Community:
https://www.figma.com/community/plugin/1254869452790045259/bitcoin-ui-kit
And the source code is here:
https://github.com/GBKS/bitcoin-wallet-ui-kit/tree/main/figma-plugins/bitcoin-ui-kit
Wanna help with the UI Kit project? Let me know.