a CSS patch improving the appearance of the tabs

This is just a cosmetic thing, but I have been tinkering with the CSS to improve the appearance of the tabs. The attached image gives you an idea of the result. The main benefit is that the bottom border of the selected tab is hidden, so that the tab appears to be of one piece with the pane below. There may be better ways to accomplish the same thing, but this works so I thought I should share.