Customizable browser other than Vivaldi

I've been searching for good browsers for months now. These are my requirements:

- It should be very customizable (and easy to customize)
- Should have extensive community plugin/theme support (if there's not good theme support it should look good out of the box OOTB)
- Preferably FOSS
- Preferably support chrome extensions (or have a large community of extensions)

Every time I've searched online I've found Vivaldi as a good option, but every time I've tried it, it's overwhelmed me with options and settings. Can someone either recommend a good browser other than Vivaldi? Or help getting started with Vivaldi?