This is not necessarily true. They just don't have the greatest documentation right now, not that they don't want you to modify. I know it's a year old but you responded - you can add !important to override JQM styles. For example, if OP would have applied font-size:150% !important as a style, it would have worked for basic text on the page.
Though widgets (listviews, buttons, etc) probably don't inherit from the body, you will have to be more specific. This means you may have to override a property in listviews with the !important command. This is very powerful when working with JQM. This way you don't have to apply inline styles on each element.