I just joined and am starting to experiment with BackdropCMS. Great project!!
In doing so, error messages came up while I was experementing with themes. I couldn't find any answers online, but I also failed in keeping records of these messages. The error messages included blank screen, DB not accessible, ...
But, sharing the solution I found might still help others.
I found that in the process of activating and deactivating themes, restoring backups and copying directories back into the installation, I ended up with two themes set to (default theme).
This was not easy to spot - because one of them was below the line of the Disabled Themes and further down, not easily noticable. I found it by "Inspecting the element" in my brower, telling me that the active style.css was of the other theme not even activated.