I am using UltraModern UI for my NSIS installation. I noticed a few design issues, which spoil all this fancy thing:
Description of standard screens are either blue or all out of sudden white. Built-in list of startup folder items is white, while the same list for files being installed is blue.
I think Start menu page is created with plug-in so you need to pass some parameter to this label [static text] to redraw it correctly - try SetCtlColors.
Or simply the ID of ListView and label are switched so listview is updated instead of label.
Try to switch them back.
Did you think about changing from UMUI to something else?