How can I change the background color on my ecwid photos?

Hello Im working with ecwid the shopping cart program , Iv uploaded some photos of my products and categories and cant seem to customize the background using css. Everytime I enter a css code for an element , nothing happens

I have tried this:

.ecwid-responsive table.ecwid-productBrowser-subcategories-mainTable
 { background-color: black}

please go here to see exactly what Im referring to: hair.hughesjobs.net

via Chebli Mohamed