How do I change the dropdown menu background color in Squarespace?

The dropdown folder menus in Squarespace 7.1 use the .header-nav-folder-content selector. Override the background on this element to change the dropdown panel color.

You'll probably want to update the dropdown link colors and hover states too, so they're readable against your new background. The dropdown items use .header-nav-folder-item as their selector.

.header-nav-folder-content {
  background-color: #1a1a2e !important;
}

.header-nav-folder-item a {
  color: #e0e0e0 !important;
}

.header-nav-folder-item a:hover {
  color: #ffffff !important;
  background-color: rgba(255, 255, 255, 0.1) !important;
}