User Styles & Stylish
I experimented with User Styles when I was using the old NWN Vault, because their look and feel was terrible, the vault has gone now and the code i.e. the customisations are held on the browser’s disk, so a growing use of cloud computing challenges this as an answer, but overwriting the CSS can be quite cool.
See https://userstyles.org/ is the repo, it has several browser add-ins, including Firefox.
See also https://davelevy.info/wiki/using-userstyles-org/ which talks about my projects and was updated in June 2021.