Thanks for your comments!
I've got it working now, but I had to modify the Twilight.tmTheme. I also stumbled upon this post when I searched for the scope name and the changes appear to be differences in the weighting of the selectors. If anyone is interested I changed the following lines in Twilight.tmTheme:
Line 350: <string>source.css meta.selector.css entity.other.attribute-name.tag.pseudo-class.css</string>
Line 361: <string>source.css meta.selector.css entity.other.attribute-name.id.css</string>
Line 372: <string>source.css meta.selector.css entity.other.attribute-name.class.css</string>
to make the scope selectors more specific (at least, I assume that's what I did..!)
Thanks for the pointers!