Our Discourse was updated yesterday and a few hours later two users informed me their preferred Light theme was replaced by Shades of Blue (which had been made the default). They switched back and forth between the two but are stuck with Shades of Blue.
When I checked in the Dashboard the first thing I noticed was the forum was updated 7 hours earlier - when both users were off-line. When I checked the Themes, the color scheme for Light is blank! I think what happened with the update is the Light theme color scheme was cleared so she was defaulted to the Shades of Blue theme… or the Light theme grabbed the Shades of Blue color scheme as something to replace the blank scheme it now has.
All the other Themes work without any problem. If it was only one user I would say she changed her preferences, but there are two users involved. I believe all my other users are using the Dark Theme and Shades of Blue. One may be using Vincent.