Improve contrast for mobile search

WCAG2AA
Discovered with [HTML_CodeSniffer](http://squizlabs.github.io/HTML_CodeSniffer/)

I changed the label color to #fff as recommended, but changed the :before icon back to its original color. According to snook.ca [Colour Contrast Check](http://snook.ca/technical/colour_contrast/colour.html), the mobile search form was already of sufficient contrast.

Nonetheless, I did also slightly increase the contrast of the search input border and placeholder text for mobile.