Navigation theme is not selected by default

Summary

When I open the preferences page to choose a navigation theme for the first time, I can't see which theme is selected right now.

Steps to reproduce

Just open the page: http://your_host/profile/preferences

What is the current bug behavior?

No theme is selected.

What is the expected correct behavior?

The default theme should be selected.

Output of checks

GitLab Community Edition 9.6.0-pre 297dfe6