Hello,
One thing I find lacking in Bear is the ability to customize colors…
It would be really cool to have a “custom” theme included in the themes that allows us to modify the colors directly with hexadecimal.
Also, being able to customize our own colors for H1 to H6 titles would be greatly appreciated as sometimes the notes are hard to read 
Thanks to the dev team and long life to Bear !
This has definitely been asked for already. Looking forward to when they do release it.
I literally just signed up to create this feedback. I love the OLED black . . . But don’t love the Yellow it comes with.
I have the same issue — existing themes are too high contrast and too bright for me, so I ended up making my own theme. The only annoying thing is that I need to modify the files inside the Bear package after every update, and it only works for a Mac.
Feel free to try it, here are the instructions.
I do want this, but I would like it to be more streamline and user friendly, guided by the app itself. For example, in Obsidian, it’s a hassle to change your theme. You change it, go back and change it again-- keep tweaking until you like it. But then you realized you changed the body font instead of the headers… and back you go to change it.
I like Bear’s current play on themes. Just click and it changes it for you. Maybe add options as a dropdown to different header colors and let you choose from those, similar to how easy it is to change themes