UI themes started out not supporting dockable windows in PB 2019 GA (ref 1).
UI themes has extended in other directions since dockable windows are obsolete as of PB 2019 R2 (ref 2).
Snippets from documentation:
. . .
What cannot be set by the theme
* The theme is not effective to the dockable window.
. . .
Ref 1: PowerBuilder Help > Users Guide > Working with Targets > Working with Targets > Specifying application properties > Specifying a theme for the application UI > Understanding what can be set by the theme > Windows and User Objects
Dockable windows are obsolete
Dockable windows become obsolete at the release of PowerBuilder 2019 R2. Obsolete features are no longer eligible for technical support and will no longer be enhanced. Customers who use dockable windows in their applications can choose to continue using the feature without support or migrate to normal floating windows.
Ref 2: PowerBuilder Help > Objects and Controls > System Object Properties, Events, and Functions > Window control
HTH / Michael
We considered dockable interface, in addition to themes, as an easy and cheap way to upgrade the look and feel of our applications...!
Depending on how you structured your code around dockable windows you may still deliver such UI though more coding required when faced with just MDI vs. SDI in the system classes.