If I were you, I'd download Firefox, its Web Developer Toolkit and Firebug add-ons.
Those add-ons allow you to examine and CHANGE the css of any web page as you view it; you can also save those changes to your own harddrive ... and then you can upload it to your css directory.
In other words...
- load up your site
- go to the page you want
- edit the css on the fly
- when you get the result you want, save the stylesheet
- upload it
- refresh the page you're working on
- if not quite right, go to 2
- move on to next page
You perhaps need to get to grips with CSS; there're tons of tutorials on the 'net; a good book is Designing without tables using CSS from sitepoint.com
As well as CSS, you can move blocks of code around in the tpl_ files - and you need to study the includes/templates/YOUR_TEMPLATE/common/tpl_header.php for your header question. If you've already been playing around, it may be better to start again with an untouched copy of the file.
Regarding your left and right edges - you're thinking the wrong way round; what you do is give the body a background colour/image and then the other classes (e.g. #contentMainWrapper) a different background.
If you start using Firefox, do remember to check what the pages look like in IE6 and 7, and if possible, Safari, Opera etc.