OK, to start with Contribute is meant for site updates where you set access rights to certain areas of a site pages. The web designer designs the site and locks areas of it out (header, navigation, footer etc) and Contribute users get to edit areas of the template that the designer allows them to. That's what Contribute is for - it isn't really for designing a site from the ground up.
Now, there is a lot of confusing code going on in there. You have <p>'s <div>'s and <span>'s doing all kinds of everything and it is pointless specifying styles for some of your layout and then using style snippets inside the body code, like this:
<p align="left" style="margin-top: 0; margin-bottom: 0;">
Basically the <div> tag should contain a class= or id= style link. Currently your <div> is wanting to align=right while the <p> tag inside the <div> is saying go left.
Odd, although not impossible to achieve.
Anyway, you can set alternate row colours by using, you guessed it, a style.
These two styles give different background colours, as you should be able to see.
If you create a series of page <div> tags to contain each cat's thumbnail image and text, assign the style to each cat's row on an alternating basis.
<div class="row1"><p> tag if you must use one, followed by details of cat 1 and picture...</p></div>
<div class="row2"><p> tag if you must use one, followed by details of cat 2 and picture...</p></div>
Note that I have used some pretty odd colour selections for the two row styles. I just wanted them to be wildly different to demonstrate a purpose.
Try this link for a very simple example, knocked up in a couple of minutes:
Here is the file as a Zip archive for you to download, if you want to:
Note that it is valid HTML and not XHTML, but it should serve as a basis to demonstrate the concept far more than my waffling on about it in here.
Stylesheets are great, if you understand them. If you don't you have two choices - either learn about them or don't use them. I'm being brutally honest here and for good reason.
Take alook at the sample file to see a concept working. You can make the page do whatever you like using styles, or just by using plain vanilla HTML tags to hold your fomatting.
You could even, if you wanted, make a multiple row table and just asign a different background colour to each rows cells, like this:
And the Zip file for you to play with:
I think you are overcomplicating what should be a simple concept. Take a break, have a cup of good coffee and rethink your battle plan.
Hope that helps a bit and gives you some ideas.