If you float elements on a tab the bottom part of the gray background will vanish. You can amend that with the following CSS:
#headertabs.jquery-large.ui-widget { float: left; }
However, as the tabs themselves are also floated left they might not display correctly (especially the most right one). Setting a width for whole tab bar seems to solve the problem.
ul.ui-tabs-nav { min-width: 700px; }