Newbie to Community, help and advise needed pls


New to elgg and finding it hard to find basic info. A couple of questions:-

1. I am trying to change the header colour of a theme and can't seem to find the css file where this is declared. I am tryign to use firebug but maybe that doesnt workj with elgg. Also want to use my own icons and can't even fidn where references to png or jpg files are.

Applogies if these seem basic questions. I ahave looked on youtube but there is only really help with installing. Any pointers or help much appreciated. BTW I have migrated away from Drupal :)