Global Look and Feel Cannot be Applied to Individual Spaces when Documentation Theme is Set on Global.

Steps to reproduce:

1. As default you're using the default documentation in the Global Look and feel
2. Go to Browse > Confluence Admin > Themes and change to Documentation theme
3. Now go to a space, that you're sure that has the Global Look and feel theme.
4. You'll see that the theme was changed.
5. Go to the Space Admin, and try to change to Documentation theme' you'll see that the theme will not change.

Workaround
Changing the Global Theme on your instance to "Default Theme" should do the trick of restoring the ability of configuring the sidebar of any space using Global Look and Feel.