Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for many popular sites.
This is one of my favorite extensions for Chrome. I use it most for one reason, my Blue Scroll Bars style: https://userstyles.org/styles/64318/blue-webkit-scrollbar. This style changes the scroll bars in things like TweetDeck, Gmail and Google+ from hard to find light grey to a nice shade of blue.