Invert color and text color of active tabs for default theme.

On many displays and configurations it is hard to differentiate between an active or focused tab and non. By inverting the tab:active widget gradient/shade/text CSS for the default theme, you not only save time but keep that task in focus in any field of vision. On long file name tabs in Scratch, any site with a fav-icon in Midori, or in the dark Pantheon Terminal, I could see this being incredibly helpful.