When using Breakdance you have two options. Use it without a theme and design everything from scratch – in that case Breakdance will completely disable your theme. This is happening by default.
In case you want to use a theme you can change this setting in Breakdance > Settings > Theme. If you want to be able to use a child theme to override a plugin PHP templates but want to keep the benefit of not having your theme enabled – most likely due to performance reason – you can use this theme: Breakdance Zero Theme with the Keep Theme option enabled.
Or download it here.
Source: What Theme To Use