The image for that is in css/ui/jquery-ui-1.8.12.custom.css
Code:
.ui-widget-header { border: 1px solid #2b567f; background: #2b567f url(images/ui-bg_glass_25_2b567f_1x400.png) 50% 50% repeat-x; color: #cccccc; font-weight: bold; }
hm for a new skin, this is not optimal at all. If you remove class="uiform" (class that spices up forms) from from the form in join_form.html the defaults from screen.css kick in. legeld, fieldset etc.
The same can also be done in the other html files if needed
Need to discuss this with mark. A way to have the pimped forms individual for every skin