Project setting gui/theme/custom has no effect on Dialogs

[andrea-calligaris]

Godot version

3.3.4.stable

System information

Ubuntu

Issue description

Changing gui/theme/custom in the project settings, has effect on the theme of most Controls, but has no effect on Dialogs.

Other problems with this setting option:
#33902
#36614

Steps to reproduce

• Add a theme resource to your poject
• Set the theme in the project settings (General → GUI → Theme → Custom)
• Make a script that popups a Dialog
• Notice no difference in the theme
• Manually add the theme in the theme property of the main control in the scene or in the dialog itself
• Notice immediate change

Minimal reproduction project

DialogsProjectThemeBug.zip

[Goutte]

Still happening in 3.4.4.

Some properties are handled (title_height), and some aren't (title_font, panel).