When you change themes, you lose Customizer settings and may not get them back if you switch back to the previous theme, so it's a good idea to export your Cutomizer settings before you switch themes. Or you might want to import your Customizer settings from one site into another site.
You can export or import Customizer settings using a plugin developed by the Beaver Builder team. This lets you either save the Customizer settings for a particular site or import the settings from a .dat file that you previously exported from the same or another site using the Beaver Builder Theme.
If you're using the Beaver Builder Theme, you can install this plugin right from the Customizer panel. If you're using a different theme, see this WP Beginner article about how to use this plugin to export your theme's Customizer settings.