f I could make these changes in style.css, I'd be happy to do that instead.
You won't be able to make these changes to your desktop theme. As soon as you switch to another theme (like Jetpack's Mobile Theme), anything you may have done in your previous theme is ignored.
Instead, I would recommend that you create a small plugin that would load only in Jetpack's Mobile Theme, and include the CSS changes you want to see on Mobile. Here is tutorial:
http://jetpack.me/2013/06/27/customize-mobile-theme/#throughactionsandfilters