Mobile custom Css

Hey im using the mystile theme and am curious if anyone here would care to explain how I’d tweak the style settings for viewing on a mobile device. For example my site http://www.prowilderness.ca
When viewed on a cell phone (not a tablet) my navigation menu is converted to a drop down style menu which I’m fine with but since I’ve re coloured my entire site you are unable to see the links within the menu since both the color of the font and background are both white. I’d really like to change the menu background colour to a dark blue.

If anyone has some insight it would be much appreciated.
Or even an app like Firebug that I could use to inspect via mobile phone.

Thanks.