A survey administrator can choose a template for her survey.
In the dropdown list "Template" in "General survey settings" she will see only those templates, a superadmin has given "template permissions" to her Limesurvey account (or she will see all templates, if the superadmin gave her account the "Permission to view/read templates").

So lets say she chose the template "fruity" for her survey. She is not allowed to modify this theme, even if the superadmin gave her account "Permission to create, view, update, delete, export and import templates", because "fruity" is a standard theme/core theme ("If you want to modify it you can extend it").

Nevertheless she can change the "theme options" of this core theme: "Survey settings -> Theme options -> Simple options -> Inherit everything: No". This is even possible with no "Permission to create, view, update, delete, export and import templates" at all.

I think that a survey admin normally expects a standard theme/core theme (like "fruity") to be unchangeable. So, if you choose "fruity" then you expect to get the original "fruity" theme and not a theme that was edited by some other user.