Give sub-menu an opacity

Take the template 1 of the drop down menu as an example. You can add a few lines to the CSS file:

#ddmenu div.drop
{
    ... ...
    opacity:0.9;
    filter:alpha(opacity=90);
}

The main menu's border will be visible due to the semi-transparent sub-menu. You can overcome this by giving a colour to the main menu's border-bottom:

#ddmenu ul
{
    ......
    border:1px solid #CC0000;
    border-bottom:1px solid white;
    ... ...
}