por Ken Task.
Without being able to see or know the theme, one can only guess ... soooooooo ....
In a stock MDL 2.3.x afterburner theme as an example:
renderers.php: $content = html_writer::start_tag('ul', array('class'=>'dropdown dropdown-horizontal'));
and in /theme/afterburner/style/ there are a bunch of .css files.
afterburner_menu.css
file has many entries for dropdown.
A custom theme might be based upon one of the stock themes.
One could install FireBug in FireFox and inspect the custom theme.
And for a CSS reference:
http://www.w3schools.com/cssref/pr_text_text-align.asp
'spirit of sharing', Ken