Crossbrowser mobile-friendly theme help

I’m trying to make a responsive mobile-friendly theme from scratch. My main target users would be low-end phones and pc users.

One thing that’s perplexing me right now is when I view my theme on a desktop (Firefox and Chrome) and Opera Mini, it works fine. However, if I switch Opera Mini to mobile view, the divs gain added margins on the top and bottom. I already set the margins and paddins to zero in the CSS file.

I tried switching to a Xhtml mp doctype to see what happens. The margins were gone, but the background colors of the divs won’t show on the desktop.

Any suggestions?