Hi Guillermo. You would use a tool like Firebug in Firefox or Google Developer Tools in Chrome. Right-click on your page on, for example, the menu. Then select “Inspect element” from the pop-up menu. This will open a window at the bottom of your display with the related line of HTML code highlighted. For each element you can see the assigned ID (id=””) and Class (class=””) names. When used in CSS, element IDs are prefixed with a pound sign (#); classes are prefixed with a dot (.).
Each menu level below the main menu has a class of “sub-menu”. This means that your code won’t be applied because the class of “.sub-menu” would apply to every level; there wouldn’t be any “ul” elements without that class. The code Tom posted above will target the 1st level of submenu items. Are you trying to change something else?