Topic on Extension talk:Header Tabs/Skin modification

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;
}