It is beyond the scope of TST project. Today TST is just a sidebar panel (due to Firefox's restrictions) and Firefox's horizontal tab bar is not controllable by addons anymore. Instead you need to use userChrome.css: https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#for-userchromecss
P.P.S. Your quiz makes no sense, shouldn't it instead say 英語で「レインボー」とは何ですか？ （小文字7文字）
It looks little unclear for native Japanese talkers. Anyway, this page is just left for some historical reasons and not a good place to track topics related to specific products. Instead I recommend you to see discussions and issue tracker of TST's project:
https://github.com/piroor/treestyletab/issues
https://github.com/piroor/treestyletab/discussions