Heya! :D I took a look at the code here on the page and everything seems to work even when switching everything to nvl. Admittedly there wouldn't be anything added to gui.rpy regarding nvl text (only the sayseen color and toggles), so the rest should all be under NVL Screens in screens.rpy. Did you also add the "nvl_chosen" and "nvl_chosen_text" styles from screens.rpy? That's where the color changes should be defined and should happen.
If all else fails, feel free to DM me your code so I can take a crack at it and figure out what's happening on your end