Hi klose,
Welcome to lilypond. You can change pretty much every single thing in lilypond, one way or another. It admits of vast flexibility - unparalleled in fact.
Our colleague here Abraham Lee (tismist) has made some very fine and eminently usable engraving fonts. You may care to check them out. It used to be difficult to adapt lilypond to use different fonts, but that is in the past and it is simple to drop these fonts in.
https://sites.google.com/site/tisimst/lilypond-fontsThe point being, these fonts provide clefs and accidentals as well as noteheads and so forth.
Apart from using supplied fonts, lilypond also allows custom definitions of noteheads, which is what I use in my work. My noteheads have a very specific shape and ellipticity that matches the handwriting of the composer I work with, and this cannot be found in ready made fonts. I mention this as an example of how customisable lilypond is in relation to visuals. As has been mentioned, you can fashion your own clefs as you will, but that's not entirely straight forward, as it requires internal knowledge of how lilypond makes and uses fonts - but it can be learned and done.
Andrew