:root {
    --dark-bg-color: #2c3e50;
}

/* 1. Sets your new main background color */
html.layout-authenticated, 
html.layout-authenticated body, 
#layout {
    background: var(--dark-bg-color)!important;
}

/* 2. Makes the internal containers transparent so your color shows through */
#layout > div,
.sidebar,
.listview,
.boxlist,
.scroller {
    background: var(--dark-bg-color)!important;
}