The menu background is missing in the css file, thus menus have no background. This is an issue only if the window itself is set to have a transparent background (like, for instance, gnome-terminal with the relevant patches to enable transparent background).

The following change in Adwaita/_common.scss fixes this issue :

*** 1388,1393 ****
--- 1362,1368 ----
  .menubar {
    -GtkWidget-window-dragging: true;
    padding: 0px;
+   background-color: $bg_color;
    box-shadow: inset 0 -1px transparentize(black, 0.9);
    &:backdrop { background-color: $backdrop_bg_color; }
    & > .menuitem {


Note : not tested with > 3.14, may be affected or not.