Theme settings can now be stored as configuration in code.

Themes no longer include any settings (or settings pages) by default. If a theme provides settings in its .info file (as you would in D7), then those settings are used as defaults.

If a theme settings page is saved, a [theme_name].settings.json file is created in the active config directory.

For more information on changes to theme settings, see the related change notice at https://api.backdropcms.org/node/26863

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/130