Just a few days ago Henrique posted his Bear themed Custom CSS. I only saw it because I was posting to ask how to hide scrollbars. I wish I had seen it earlier as it would have saved me a job! As it happened, I was in the middle of producing a Bear theme myself, which I have now finished.
I'll make the same disclaimer as Henrique, my CSS is probably a mess. For example, I have a lot of '!important' in it. If you know better, and can see things I've done wrong, please let me know how to rectify it. I'd like to have decent CSS.
Screenshots of my Bear theme are below. I have Zettlr in the 'Karl-Marx-Stadt' theme giving red colouring. I also copied Bear's welcome page to give you a better comparison.
This first screenshot is very close to Bear. It's minimal with no toolbar or tabs and uses 'Night Mode'.
This second screenshot is the same minimal Bear theme with 'Night Mode' off.
The third and fourth screenshots are the same again with the toolbar and tabs returned.
My custom Bear CSS can be found here. Enjoy!