Can't find the style controlling this roll over state

My question was asked here:

I have been searching for the style that is controlling the "red" hover on the thumbnails in the "Latest from the Blog section" on the home page in my theme.

Ash suggested this code: .side-list li:hover img{
background: #000000 !important;

I added it to the Custom Styles under theme options and it's not working. Any help would be appreciated!