Pleroma configuration

Also, is there a way I can customise my instance as if it’s a website (e.g. modifying css style sheets, etc.) so that I can modify things that are beyond simply changing the background image or my avatar?

For example, see the example status text (“I did the thing”) and the button label (“PULL THE TRIGGER”) in this screenshot from cyuucat.neocities .org showing shitposter.club — I presume this shows a from-source installation.
The defaults are “Just landed in L.A.” and “Post”.