You can use TVs to do this. If you are using background images and colors with CSS, put the changing parts in a style block in the head of your template using the TV tags as variables for the values. Then edit the TV for each document. First, of course, you will need to learn how to use TVs!
http://modxcms.com/template-variables.html
Here is an example of one template where I change colors depending on the section of the site:
<style type="text/css" title="currentStyle" media="screen">
body{
background:#[*Background*];
}
#banner3 li.here, #banner3 a:hover, #headerleft span.here, #hmnTopMenu li li a:hover, #content h1.colored{
color:#[*Background*];
}
</style>
All I do in the TV for each document is specify the rgb value I want.
I used the @INHERIT binding as the default value, then all I had to do was set the rgb value in each section folder "parent" document, and all its children automatically get the same color.