I've put an example of doing this type of task on my blog, which you can view here.  The key part is including your item elements in a div tag, and this little snippet of xsl, after the div tag and in the area where you are getting your feed items:

   <xsl:if test="position() mod 2 != 0">
    <xsl:attribute  name="style">background-color: #F5F5F5</xsl:attribute>
  </xsl:if>

The example on my blog shows the entire xsl.

Sure, no problem. Basically, we are testing the item number's position in the list, which counts from 1 to see if we need to change the background color.  It works like this:

  <xsl:if test="position() mod 2 != 0">
    <xsl:attribute  name="style">background-color: #F5F5F5</xsl:attribute>
  </xsl:if>
 

The first time through, 1 mod 2 = 1, so the color of the background is changed.
The second time, 2 mod 2 = 0, so the test fails and the default color is used.
Third time through, 3 mod 2 = 1, the color of the background is changed.
Fourth time, 4 mod 2 = 0, so the test fails and the default color is used.

and so on...

This results in the even numbered items to use the default color of the container used and the odd number items to use the alternate color.  You could vary the action by modifying the test to:

<xsl:if test="position() mod 2 = 0">
 

which would result in the odd number items to use the default container color background, and the even number items to use the alternate color.

You could further modify the snippet to include an additional test to specify another color be used for both.  Take a look at TopXML's site, or just google on XSL for some other possibilities.  DNN has a nice XML/XSL module that can be used easily to explore the possibilities, or take a look at the xSelerator xml/xsl editor application, which I mention in my blog on Useful Developer Tools.