#bookmarksMenu menupopup .scrollbox-innerbox {
  background-color: #dff !important;
}

I think you mean the "classic" Bookmarks menu on the menu bar, and not the newer style bookmarks menu which opens from the toolbar button (the one that looks like a bit like a clipboard and is joined to the star icon). There is no built-in feature to change the background color of those menus, but you can do it using a custom style rule, or perhaps an add-on. I realize the following is gibberish to most people, but it can be used to apply a new background color to the classic Bookmarks menu using either a userChrome.css file or the Stylish extension. If you don't find a more convenient method, a volunteer can give more details: #bookmarksMenu menupopup .scrollbox-innerbox { background-color: #dff !important; }