If i don't make error : user have always read right on default template.

If that's the case, then the option under "Edit theme permissions" is misleading (should be grayed out or something like that)
What I found out in the meantime:

When I have "vanilla" set as global default theme (https://manual.limesurvey.org/Themes#Global_setting_default_theme), I have the options as discribed above.
When I have the Theme "Template A" set as global default theme, I have only the option for that Theme.

It looks like the User (even when the Permission is not given) always has the permission to choose from either the Templates/Themes he has the permissions for or the global set Template/Theme
SO I don't know if that's actually a bug or expected/wanted behaviour

@c_schmitz Thoughts?

If that's the case, then the option under "Edit theme permissions" is misleading (should be grayed out or something like that)

Yes : need a sentence somewhere.

In my opinion on new surveys only the permitted template should be selectable.
For existing surveys the already set template should be also available, even if it is outside current permission (so the user can still edit a survey).

In my opinion on new surveys only the permitted template should be selectable.

Right : i just check : in 3.X
As user (create survey only)
default theme is used when create
Theme list is empty except default one

As super admin
Set another theme
Save

As user
Current theme is shown, list is empty.

User can not update theme, and when create : default theme is used.

Check same in 5.X ?