XF doesn't really operate on a ton of static .css files you upload to the webserver directly. Check out this document here:
https://xenforo.com/docs/xf2/styles/#colors
Appearance -> Style ->
your style -> Style properties will let you define your colors/palette without having to modify a bunch of CSS directly.
Once you have your palette, click on Appearance -> Style ->
your style -> Templates and start tweaking the .less templates to adjust accordingly.