1. Use Web Inspector or Firebug to examine the page and it's CSS.
2. You can experiment directly in either of these and modify CSS until you get what you want, then you can change your CSS file. It's common to load a CSS file of your own after jquery.mobile.css in which you perform overrides.
3. If you don't know how to use Web Inspector or Firebug, learn. It is essential to working with JQM. (Or web design in general.)
4. (At risk of backlash...) We are not Web Monkeys here to do your work for you. If that is what you want, that is what rent-a-coder, etc. are for. You really need to learn the basics. We can point you in the right direction, and if you come across a tough problem, we all love to debate and get to the bottom of it.
You need to remove the bottom border and background of the header, according to what you have posted. You should be able to do that with the above advice.
If you want this for all of your pages, you can take care of removing the background in Themeroller.
