Users being allowed to select which theme the site uses is uncommon, and this feature has been removed from core.

If needing to restore custom theme per user, then hook_custom_theme() can be used to restore this functionality.

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Architects, Administrators, Editors
Module developers
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/256