Updated to match Chrome's Setting page a tiny bit better

* Changed default text color to #303942
* Made links have an underline by default, and disabled it where needed
* Made header background slightly transparent
* Increased .content padding-top by 5px
* Changed button:disabled to have cursor: default;
* Fixed the .close-button image. I got the pic from http://goo.gl/JOaN39
* Added a pic for .close-button:hover