Thunderbird is basically an Outlook-from-fifteen-years-ago clone, and I've always disliked Outlook, even before the recent push to make it even worse. Everything I disliked about old Outlook is exactly the same in Thunderbird, except the licence.

I don't want much from a mail client, just:

• basic stuff works
• I can see an unread email count for each of my accounts at the same time, and also have the list of messages for the account I'm looking at and a reading/writing pane at the same time, too.

Thunderbird and Outlook will only show the unread count once you've expanded the list of directories in an account, so once you've got more than two accounts with a reasonable number of folders, any further accounts end up pushed off the bottom of the screen. This isn't something that a theme for Thunderbird can change. It'd be a small change to include a total unread count next to the list item for each account when it wasn't expanded and a total unread count next to the combined inbox button, but I'm not maintaining a fork of a mail client myself when it'd still be too Outlook-like to avoid being annoying.

In the end, I settled on Mailspring, but it doesn't score brilliantly on the basic stuff works bullet point.