Make working with themes / css easier
I like that I can edit the CSS, but it takes a lot of work.
There seems to have been a selection of themes in v1, and on https://protonmail.com/blog/protonmail-themes you write "stay tuned for updates regarding themes in ProtonMail 3.0."
I found https://amdelamar.com/pm-theme, which should work with v3 (haven't tried them myself), and Detchem from here https://github.com/scastiel/pm-inbox-theme still mainly works, but the CSS is too long for the few things I want to change.
Please make working with themes / CSS easier and/or add a place where people can post their CSS and screenshots of how the result looks.

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ms commented
More recent version of Detchen: http://scastiel.gitlab.io/protonmail-theme-creator
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ms commented
PS Votes for a dark theme here: https://protonmail.uservoice.com/forums/284483-feedback/suggestions/13005834-dark-theme-would-be-ideal