The color of the outside border or the color of the form? That section is wrapped with the page div class. If you need to find out what handles what on certain pages, the best thing to do is look up that template and view the classes used.
This line in the "register_rules" template should be editted and do the trick for you:
HTML Code:
<div class="page" style="border:thin inset; padding:$stylevar[cellpadding]px;<if condition="!is_browser('konqueror')"> height:175px; overflow:auto</if>">