I'm not sure if this is possible at all, but just in case that it is: the gap when switching between workspaces is not themed.
Screen-recording example below.
https://github.com/user-attachments/assets/358214a2-66fc-4f1c-a27b-507adfdb4f38
Theme in use: Catppuccin Mocha
Rewaita 1.1.1 flatpak