styles/xf.theme.dark.less (7 lines of code) (raw):
/* Overrides specific for Dark theme
===================================== */
.xf-theme-dark {
}
.xf-header {
.border-box;
border: 1px solid #212325;
.box-shadow(~"inset 0 1px 0 rgba(255,255,255,.2), 0 1px 1px rgba(0,0,0,.55)");
}