Sub-theme of a sub-theme
You always start with a base-theme.  Let us call this Theme A.  If you introduce a sub-theme (e.g. Theme B as a sub-theme of Theme A), it inherits everything from Theme A.  You can introduce minor (or major) tweaks in Theme B.

If you set Theme B as the default, then you get Theme A + Theme B.

If you set Theme A as the default, then you get Theme A (with nothing from Theme B).

In other words, if you've set Theme A as the default, then Theme B is not supposed to work.   To get Theme B to "work", you must set Theme B as the default.

This also works if you cascade with more sub-themes.

For instance, if you introduce another sub-theme (e.g. Theme C as a sub-theme of Theme B), it inherits everything from Theme A and Theme B. You can then introduce minor (or major) tweaks in Theme C.

If you set Theme C as the default, then you get Theme A + Theme B  + Theme C.

A theme need to be enabled to be set as default, but works fine as an inherited theme even when it is not enabled.