Choipadua17 I'm not on Discord. I think I signed up once, but haven't been back since.
Like I said, finding various CSS rules and playing around with them isn't too hard with a little practice and research. Of course, it also could be that you already know how to do it. I'm not sure what your CSS experience level is like.
Here's one tutorial on how to use Chrome's developer tools to inspect CSS/HTML code. Once you know what the elements are called, you can start changing the rules from there.
https://designtlc.com/use-chrome-inspector-edit-website-css/
I hope this helps. Plus, by not using my exact changes you may end up creating something that looks even better!
One more tip that I'll give is there are separate CSS rules for small screen sizes, like on mobile devices. But, Chrome's developer tools will also allow you to go into mobile view mode. This way, you'll be able to also inspect the CSS Flarum uses for small screens. You don't always need a separate rule for small screens. But, you will find that certain elements/containers do have separate mobile CSS rules.
Happy tweaking and designing! Half the fun of it is figuring it out and putting your own spin on the styling.
Namaste.