Hi Melissa,
Firebug is showing you what styles apply to that element and where they are. In this case
.moodle-dialogue-base .moodle-dialogue-wrap on line 5 of one of the yui css files (yui_co...ion.css) not the yui_combo.php.
But - do not change it there! If firefox shows you that the css rule comes from inside your theme, then you are ok to change it in the location it tells you. But if the css rule comes from a parent theme, or in this case from the yui css files which are part of the moodle core, then you should add the rule to your own theme css files rather than editing the parent/core style sheets. Adding them to your own css files will overwrite the rules coming from other parts of the system like the parent/core styling.
So in the example you've given adding
.moodle-dialogue-base .moodle-dialogue-wrap {background-color:#0000ff;}
to your moodle/theme/yourthemename/style/core.css file would give the section a blue background instead of the white one set by yui css. (Obviously you use the rules that you need, the background colour is just an example as it was one of the styles showing in your image )
HTH
Richard
EDIT: Just adding to Richards comments here to make it clearer where to find the CSS file in your Moodle intall
(Edited by Mary Evans - original submission Thursday, 30 June, 201102:26 PM)
(Edited by Mary Evans - original submission Thursday, 30 June 2011, 03:22 PM)