Setting the theme for a page
Use data-theme:
<div data-role="page" id="Page2" data-theme="a">
Note that the headers and footers will still default to theme "a". To change them you need to specify data-theme="b" in the header and footer divs too.
Setting Theme of An Element
data-theme="a"
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.