ok I understand now, unfortunately there is not an easy way to do this, as there is not a generic ID available for assigning a CSS class.
Each menu item however has its own unique ID, if you use the web developers toolbar you can use the toolbar to identify all of the ideas in the menu. ( checkout: Troubleshooting DotNetNuke Skins
The only option is to create a separate skin for each of the main parent tabs, and then within the skinname.CSS file add some CSS to target the unique ID of the parent tab. - This is the only way I think you can do it and it's pretty messy and time consuming
Your other option could be to display breadcrumbs which will allow the users to easily identify their location without needing to highlight the parent element, thanks