you can do it by changing the parent theme in the XML file, and then by reseting the theme.
it could be possible to do via the admin interface too: but we can't guarantee that changing the parent theme will always work. So it should be only for expert users who knows what they, so it's ok to keep it in the XML.
I'll write a note about it in the wiki.

https://manual.limesurvey.org/New_Template_System_in_LS3.x#The_manifest_config.xml

You mean only byy FTP : yes i know we can already do this … this feature request is for Admin GUI …

I can always do anything via FTP even delete all core template … and set it to only my template (maybe there are issue here)

for Admin GUI: I'm against it, because you can't guarantee that the new parent theme will be compatible with your changes. So it's only for advanced users....

«Maybe track down final theme is it's the same.»

And currently : you can easily broken anything in advanced options … just adding "parent template" don't broke more.

Else : i can make a new plugin to do this …

And more : in general user only update one or 2 files, maybe just custom.css … and here : this can't broke anything

yep it's true, theme editor is already an adavanced feature...
so why not.