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