How can i setup lithium that it uses my own css file? And is it possible to use lesscss (http://lesscss.org/)?
Are you talking about Lithium, the top-notch framework for bad-ass developers ?
If so, it's impossible ! Lithium styles are so sexy! Why the hell a developer would change those ?
More seriously, how can you ask this question ? :-) Have at least read something about Lithium ?
PS: on your views/layout/default.html.php there is line where Stylessheets files are included. Tweak it as you want, it's free !
PPS: and about less, sure you can ;-)