browser default styling makes mixing childless top-level items look like nested items

[sffitzpatrick]

Hi, I think this is related to #5. When <optgroup> and <option> tags are mixed in the top-level nav (i.e., some main menu items have children and some don't) a top-level <option> is visually indistinguishable from nested <option>. Attaching some browserstack screenshots.

It's not a bug in this plugin's code, but still makes the menu visually confusing. I wonder if anyone's noticed this and thought about a solution?

Screenshots show the index.html packaged with the repo.