Changing the blue color of the main theme to black

[pat_togo]

Dear All,
I have just installed SMF and would like to use the SMF default theme. Nevertheless, I would like to change the blue into grey or black, which file do I need to modify to do that?
Thanks a lot.
Pat

[1MileCrash]

http://www.simplemachines.org/community/index.php?topic=61145.msg423569#msg423569