how to contribute to lemmy/kbin UI?

I would like to try making a few minor changes to the CSS in either kbin or lemmy both of which I've been trying out. I favor a more dense interface and think styles could help visually separate different kinds of content. Maybe to reorganize some stuff if I got more ambitious.

I have some experience with web stuff, css. I can get around in git. But I am not a sysadmin or a programmer. And I have never worked on such a large comprehensive project.

Here are the repos I found:

codeberg Kbin/kbin-core

• seems to have no contib guidelines
• issues: 50 open, 4 closed

github LemmyNet/lemmy

• contrib - same link for both repos but doesn't really give much useful info

• Issues: 184 open, 1,594 closed

github LemmyNet/lemmy-ui

• issues: 197 open, 507 closed

Do I need to run the whole server with back end to fiddle with the CSS? Or can I use the developer tools in my browser to produce an alternative stylesheet?

Maybe it is just way above my skill set to make full PRs in something so complex. Should I post mock ups of how I'd like it to look as an issue?

Or just wait for someone who knows better how to get it done to come along?