I'm looking at whether I should migrate a site with a lot of custom modules from D7 to Backdrop rather than struggle with D9 (isn't everyone?!).
But one thing that is doing my head in is the default themeing, both Basis and Seven, it's just making working with Backdrop so hard. It's fallen foul of the (to me) incomprehensible modern fashion of making everything on the screen as BIG as possible - huge fonts, huge margins around every box, etc. The result is very few items on the screen at once, so instead of, for example, just quickly scanning a list of items you have to continually scroll to see everything. All that white space may look pretty to some people, but it's not the point, it's not productive.
Curiously, the admin bar drop-down is almost the reverse, it's very compact, you might even say the font is too small.
I could make my own sub-theme to create a more comfortable Drupal-7-like experience, but I don't want to go to that hassle if someone else has already done it.
At this stage it's mainly Seven that's annoying me, trying to configure a site. I will hopefully be able to port my user theme over from D7 is due course.
My overall impression of Backdrop's out-of-the-box theming is the same as that expressed by the OP.
Working toward the same end, I have been slowly developing a theme to my liking. Right now it's just a giant hack-n-patch of another theme, but the results are very encouraging. I believe it will be well received if I ever get to the point of putting it out there.