Having the same issue. Some experiments with firebug on my page with
cbox-theme/dynamic.css?ver=3.5
.top-menu, .top-wrap .sub-menu, .item-list-tabs, .footer-wrap, thead:first-child tr th, thead:first-child tr td, .base-menu ul ul, .sidebar-activity-tabs ul li.current, .sidebar-activity-tabs ul li.selected, .docs .toggle-switch, .post-title:before, .doc-content-wrapper .toggle-switch, .mobile-menu-container, .info-box h3, #bbpress-forums li.bbp-header {
background-image: -moz-linear-gradient(center top , #333333 0%, #222222 100%);
background-repeat: repeat-x;
border-bottom: 1px solid #292525;
border-top: 2px solid #525151;
box-shadow: 0 1px 3px rgba(0, 0, 0, 0.25), 0 -1px 0 rgba(0, 0, 0, 0.1) inset;
}
indicate that disabling the background-image.. line then shows the correct background color.