Hi Shell
Would you have a URL/link to an example of your screenshot and we will investigate this issue further.
You could try creating a class suffix for your module as detailed here...
http://joomla51.com/tutorials/item/applying-custom-css-to-an-individual-module. The CSS you would use would look like the following where 'grey-background' is your module class suffix (edit to taste).
.grey-background {
background-color: #cccccc;
}
Ciarán