regolith-session-flashback appears to conflict with regolith-look packages

[scmbradley]

Here's what happens whenever I try to install a regolith-look package:

$ sudo apt install regolith-look-nord
[sudo] password for seamus: 
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
 regolith-session-flashback : Conflicts: regolith-styles but 2.9.7-1 is to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

I haven't tried them all, but ayu-dark, solarized-dark and nord all have the same issue. What could be causing this?