The discussion revolves around the idea of a Theme Component Matrix to help manage theme components across multiple themes. Dannii proposes a visual matrix where themes and components are displayed in a grid, with toggles to enable or disable components per theme. justin.noom supports the idea, calling it excellent. sam suggests testing the concept first with a data explorer query and a custom theme component before adding complexity to core, also noting that the current UI already shows which themes use each component. tobias eigen confirms interest in such a tool and shares a screenshot of the current component theme list, highlighting that the UI has improved since the idea was first proposed.
When you have a lot of themes and a lot of theme components it can be hard to ensure that your components are turned on for all the themes you want them to be on for (and only those).
The idea of the Theme Component Matrix is that it would show you at a glance all of your themes and their components. In the mockup I made it just says Yes/No, but those would be links letting you instantly toggle the components.
If there are too many columns, maybe the component names could be rotated with CSS?
@Dannii This is an excellent idea!
Prior to doing anything in core maybe try 2 things:
A data explorer query for this
A theme component that adds Matrix 
I worry about adding complexity here to core.
Also on core’s side, I think it makes sense to show a list of “themes” that have a component in the actual customize menu for the component. Just need to be careful not to clutter that UI too much.
Just came across this topic while looking for such a data explorer query! Has anyone created one yet?
The UI for managing themes and components has been improved dramatically since this topic was started. The components list shows all the themes each component is used on:
