Was looking for some already built jQuery Mobile themes, and came across this great project called “Graphite” available at http://driftyco.github.io/graphite/. They’ve already built several minimalist and very good looking themes.
A demo is available on their website showing you the different theme colors you can use and allows you to get a feel of the theme through a functional showcase. Check it out since that could be useful if you want to save some time. For the purists who want to create their own jQuery Mobile themes from scratch you can use this great tool called jQuery Mobile ThemeRoller