This user script can be installed directly from https://openuserjs.org/scripts/TraderSamwise/FTX_Custom_Theme. You must use something like TamperMonkey to run user scripts on Chromium based browsers.
Script to customize the theme of FTX.com as well as display PNL in BTC. By default the script only activates on /trade pages but because react uses a virtual router it only knows the first page loaded on FTX.com, so you have to either go straight to a /trade path or reload once you get there.
Code is completely open source but I make no guarantees that it will work or be maintained. Use at your own discretion and feel free to submit pull requests.