its not straight forward to change theme as above.
jQM enhances all controls including collapsibles by adding new css classes and wrapping new elements etc....
Its the basic concept of jqm.
UI which you are seeing in jqm is not based on the initial markup, but they are enhanced.
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You can do an 'inspect Element' (hopes you are familiar with that) and see how your initial markup is changed.
I don't think there is any method exposed with which you can change themes..
only way is to toggle css of enhanced elements from js.
See below sample. Its just a sample, you must put effort to optimise it further.