Little bug in the theme.
have a look in index_dark.css:
.category_boards .board_icon, .board_key:before {
background: url(../../images/_dark/board_icons.png) no-repeat;
}
should become:
.category_boards .board_icon, .board_key:before {
background-image: url(../../images/_dark/board_icons.png);
background-repeat: no-repeat;
}