I have been using Thunderbird email for over a decade now. It is a functional email client which does its job. One area where Thunderbird can do a lot better is in its UI, I do agree that the default theme is very functional and once you get used to it you do not care.
But customization has been one of Thunderbirds strengths, so I am using a beautifully designed theme/add-on based on the original ‘Monterail’ theme. There are two versions of the theme one a normal dark and the other a full dark version. What I like about the themes is the spacing/padding usage is just about right and the content does not seem crammed, also the icons and the font used is smooth to the eyes.
Monterail Dark 2 (Add-on link)
I would call this the light theme of the two as it has a good mix of dark and light colours. I use this when I have the system UI set to light mode.
Monterail Full Dark 2 (Add-on link)
This is the proper dark theme which I use when set the system UI to dark mode.
