As someone who loves to modify the tools she uses, this was a bit of a no-brainer when I finally looked into it today.
Userstyles are custom css stylesheets you can run in your browser to affect the display of sites you visit. To use these, I'm running an extension called Stylus.

Stylus basically lets you run a bunch of stylesheets that you can easily toggle on/off, as well as lock them to specific domains or URLs without needing document queries. It's neat! This means you can change how cohost appears to you.
Here's me enabling a userstyle I threw together to change avatar sizes:
Yes I also changed the foreground colour... sorry staff I just love green
Naturally, this isn't limited to cohost, so there's a load you can do to mess around and have fun!
With that said, people on this site are coming up with neat userstyles all the time, so it's a good idea to look at the #userstyles tag if you're interested!
One thing to be aware of is that as staff keep building out the site, class/id names may change, or your existing userstyles may mess with how new features display for you as they get released. This is part of the reason why I like Stylus's ability to quickly disable styles.

