[Jon Foote]

the image is >> skin\common_files\images\head_linebg.gif
there is also an orange border that can be changed in >> skin\common_files\css\main.css
search for #header .line2
and change #cb480c to the colour ref. you want.

alternatively if you just want a single colour for the bar
change background: url(../images/head_linebg.gif) repeat-x top;
to background: #(your colour ref. here);

as mentioned above get Firefox and the firebug addon and you can find all this info easily.