For the moment it appears that we may need to remove the "light/dark" icon from the top right and exclusively use the "Control Panel/Settings and Privacy" menu to select among the themes and select whether you want dark or light style. Here's the explanation:
The light/dark icon was set up under an older version of the software before version 6. Now with version 6, all of the themes have both light and dark variations, but the light/dark icon hasn't been updated to work with the new system. Because it isn't updated, it doesn't work in most cases. I will continue to try to make it work but at the moment it's probably causing more confusion than anything else.
Let me see what I can do but if you get frustrated please just use the Control Panel/Settings and Privacy menu to select your theming. I will work on improving the situation because I too want to swith back and forth quickly.