Update common.css

A chrome-bookmark-like style. The green style may not match its surroundings enough, so some people may dislike it on the first look.

I just modified the css as little as possible. You, as the author, are welcome to modify the details to combine my suggestion and your design theme. If you want it closer to chrome bookmark bar, some fading effects, I think, are required using js.

Before you implement the self-defined css function or before you find any other css config good enough, you can perhaps adopt this css.

Among all ext's I've searched, yours is the one closest to the chrome original concise new-tab. That's why I try contributing a little to your project.

Good luck to your ext!