Another nice page on the same site -- adding Themes to Mastodon!
https://wiki.nixnet.xyz/books/masto-tweaks/page/themesskins
The author describes how to obtain a fully fleshed theme like the ones in use at Toot.cafe or Witches.Town from their Github depositories (links provided!)
And then, you could modify either of them to your own liking.
I really like the custom theme Renato Lond created and uses at his instance - https://masto.donte.com.br/
Green background is easy on the eyes, while some contrasting elements add pizzazz and make it easier to find controls and scroll bars (these, sometimes are blurred into the Dark Themes I prefer) Screen cap of the default Web client view attached.