I'm not a big fan of their new Material 3 theme. It's all very pink and orange and not enough contrast. Let's see where they'll take it.
Thunderbird
Thunderbird is the leading free and open-source email, calendaring, newsfeed, and chat client with more than 20 million active monthly users across Windows, macOS, and Linux. One of Thunderbird's core principles is the use and promotion of open and decentralized standards.
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They've got hardcoded colours in there still I think, because everything else on my system will be blue and then K9 is the only one that's pink.
I was excited (former regular thunderbird user in desktop mid/late-2000's to mid 2010's) and downloaded it but it appears to still use IMAP for email addresses on MS Exchange servers, so it's not an option for me at the moment, unfortunately.
Ah well. :(
They're getting closer, but yeah, it's not there yet.