.MainMenu_MenuContainer
This styles the main menu container for the root menu items. The default.css file uses a
background-color: transparent; This allows the image which runs underneath the menu and search section to show through. In the DNN-blue skin the image is added to the table background using the skin-gradient class.
.MainMenu_MenuContainer
{
border: 5px solid green;
background-color: yellow;
}

This is the original menu using:
.MainMenu_MenuContainer
{
background-color: transparent;
}
Note: if you remove the
background-color: transparent; statement, the image that is displayed behind the menu still shows through, so this element could be deleted without any problems occurring. (This has been tested in various browsers.)

This is the menu with the skin-gradient class in the DNN-Blue skin.css file set to a background color of red: