Mobile is designed as a mobile-only HTML5 theme with a focus on clean, readable, usable display of content and accessibility of functions. You can use the base theme, use one of the included child themes, or make your own child theme.
The intent here is to keep the theme clean, lightweight and simple. There are not oodles of custom variables, theme settings or extraneous stylesheets to manage.
Mobile 3.x branch is HTML5/CSS3, with support for most modern mobile browsers. Recommended: Modernizr to enhance support for browsers less supportive of HTML5. See http://mobilehtml5.org/ and http://caniuse.com for up-to-date info on what browsers support the HTML5 features you need.
Mobile – Base theme focusing on layout and templates.
Mobile Light – Some limited CSS for an aesthetically pleasing light-colored theme.
Mobile Dark – Some limited CSS for an aesthetically pleasing dark-colored theme.
See Branch 3.x Status, below, for important information.
Branch 3.x (HTML5)
Focus is on user-facing output. As HTML5 is evolving, so will this branch of the theme, at least until there's a stable 1.0 release.