Hello,

When previewing a new forum thread, the background color is lemon. We had the same issue with new art submission previews but fixed it in http://opengameart.org/sites/all/themes/oga/oga_no_side_bar/oga_theme_no_side_bar.css

To apply the same fix to the forums, please add

.preview .node {
background-color: #000015;
}

to http://opengameart.org/sites/all/themes/oga/oga_theme.css