DUI: highlighting toggle triangle

We found out, that triangle isn't highlighted only for root categories, that are full of classes. For others(like Operators) everything worked properly. So, I've decided to update xaml styles.

Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5087](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5087) DUI: On hovering category style is not changed for core libraries
When NC is root category and mouse is over, make background dark grey.