The fastest and easiest option that I know of to resolve this challenge using this Flatseal
Steps to set a default theme for Flatpak applications: Navigate to All Applications horizontal tab —> Filesystem group —> Other files subgroup; Add the appropriate variable(s). Redo the same for Environment group. In the future, most future Flatpak applications you open will use the new theme.
Detailed steps, additional information, and attributions at GTK4 apps do not use adwaita dark