How to edit HTML/CSS on my own instance?

Hello world!

Was wondering if there was a way to edit the CSS/HTML of my lemmy instance. It's my own personal instance, so I'd love to add links that would help me get around easier and really make my instance my own.

I have access to the entire server, and I know a fair bit about navigating debian and editing files and all that. I'm still a bit of a noob though and do not know where the header/footer/css/files are stored on my live instance.

Any help would be greatly appreciated. Thank you in advance!