Colors are all changed in the stylesheet.css file for your custom template.
To change the width, look for this section of code in the stylesheet.css file:
/*wrappers - page or section containers*/
#mainWrapper {
background-color: #ffffff;
text-align: left;
width: 750px;
vertical-align: top;
border: 1px solid #9a9a9a;
}
HOWEVER -- do NOT change your default template code! Make a new template and install it and create the changes there. Otherwise, you will be reinstalling everything after an upgrade.
The easiest thing to do is copy on your computer every folder that says "classic" and name it whatever you want (no spaces). Then go into the includes/templates/yournewtemplate/template_info.php file and change the information to reflect your new template.
Upload it all and install it in the admin panel. Now you can configure to your heart's content.
Another caveat -- the classic template width can be changed. However, I have never played with it doing what you want to do, and you'll have to change the header background graphic to a repeating kind to fill in if you want the same look. So don't be surprised if you have unexpected results when doing this.
HTH,