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I could not find one working in https://userstyles.org/.
I use Firefox and I'm open to use any extension allowing the restyling of a website, like Stylish.
I know "less ugly" is subjective, but I'm interested to try whatever attempt someone would have shared for that.
Thanks for your answer and sorry my question was unclear, I'm not looking for a theme but for a set of CSS rules that makes the whole display of the basic HTML version of GMail "better", "less ugly". I've updated the question accordingly. – Florent2 – 2018-09-26T19:42:56.640
Maybe you could use this extension regardless the permissions that asks are annoying and risky.
– Alan Garrido – 2018-09-26T19:56:26.560Welcome to Super User. As sometimes happens, a question may be unclear and you post what seems like a good solution. That makes clear to the questioner that it wasn't clear, and they refine the question, leaving the answer irrelevant. When that happens, the answer may attract downvotes because it doesn't answer the visible question. If you have a new solution for the clarified question, you can modify this one. Otherwise, it's better to just delete it. You could also look for another question for which it's an answer and repost it there so your effort isn't wasted. – fixer1234 – 2018-09-26T21:28:05.283