The "flat" style being distributed today is a giant step backwards in usability, I can't for the life of me understand why it's so popular. On desktops, it's not clear until you hover that it's even an interactive element. On mobile? Forget it. You'll have better luck teaching your grandmother the difference between client and server side JavaScript.
From a usability perspective I don't think this really works at all. For most of the UI elements, the only visual indication that you get that something is clickable (buttons, link, collapsibles) or separate (grouped buttons) is on mouse hover.<p>But this is for jQuery Mobile, and you don't get mouse hovers on the devices this is targeted for.
The vertical alignment of the text is off and the colors are a bit muddy compared to the vibrant palette of <a href="http://designmodo.com/flat-free/" rel="nofollow">http://designmodo.com/flat-free/</a> (which was clearly the inspiration for this).
I tried porting it to jQM too. <a href="https://github.com/iakshay/jqm-flat-ui" rel="nofollow">https://github.com/iakshay/jqm-flat-ui</a>. Didn't really continue..<p>jQM is for mobile devices why are you using custom dropdown?
More stuff available in the same kind of flat design style on <a href="http://designmodo.com/flat-free/" rel="nofollow">http://designmodo.com/flat-free/</a>