make sure search icon persists when input has focus

In the dark color schemes the hover background is the same color as the search icon, effectively hiding the icon when the input is in it's hover state.  This pull request will replace the light search with the dark one on hover state.  It also uses the background image off screen preloading technique discussed here: [http://css-tricks.com/snippets/css/css-only-image-preloading/](http://css-tricks.com/snippets/css/css-only-image-preloading/)