hey there! any visual style is encouraged! our upcoming adventure module has a very different look from the core book:
that said, the fonts used in the rulebook are Space Grotesk, Microgramma, and Univers Else/Fluxisch Else.
patent drawings like those in the book are largely public domain, but some are tricky so be careful when using them. there's also a lot of excellent public domain diagrams and lineart over on freesvg !