New Blog Templates CSS

Hi,
I would like to change the style for new blog templates preview page.

.theme-previewer-wrap, .theme-screenshot-wrap {
    background: none repeat scroll 0 0 #FFFFFF;
    float: left;
    margin-bottom: 25px;
    margin-right: 4%;
    max-width: 600px;
    padding: 3px;
    position: relative;
    width: 45%;
}

How can I override this code?

Thanks,
Damian