I don't think this will work... it will at most render the dropdown in bold font.
As I understand it, you want something like this: If typed in 'lor', you want the dropdown to look like:
Lorem ipsum
Lloret de mar
Lore
Lost in space
(added color for highlighting)
I don't think you can solve this with simply adding a style, you need to completely rewrite how the dropdown is rendered.
And since the AutoComplete is extended from a DropdownBox which uses a ListBox to show the autocomplete values, you have a few controls to extend...
In the end, I think you should do your css jiggerypokery in the (extended) ListBox control method 'onAfterRendering', that's where currently the various <li> objects are created
Not sure if this extra styling is worth all the extra effort though