styling customizr for dark theme

Trying to style the .thumb-wrapper element. I am using the “grey.css” stylesheet (after selecting the “grey” skin in customizr). I would like to create a dark theme (black or dark gray background) and I can change the background-color on everything (body, header, footer, nav) without trouble. However, so far I cannot re-style the .thumb-wrapper element to be black. I have tried by doing this:

.thumb-wrapper {
width: 269px;
max-width: 100%;
height: 250px;
position: relative;
left: -1px;
display: block;
overflow: hidden;
text-align: center;
margin: auto;
background-color: black;
}

but no effect upon re-rendering the page.

What am I missing? Anyone got a hint?