Popup and Search not available in Custom sub-theme

Problem/Motivation

After having difficulty migrating from d8w3css to solo, I decided to rebuild scratch.
I created a d10 site, loaded Solo and created a custom sub-theme following the guide.
Watching video - I desired to use 'popup' block - however -the block (and search block) are not available in the custom sub-theme as seen in 'https://www.youtube.com/watch?v=2JY5jYvC-T8&list=PLZVSLeSmRrd1nqYAFOyalL...'.

I enabled the sub-theme that is a part of this theme - and it is also not showing there. Does having the main Solo theme override the sub/custom sub themes and changing its value flow down?

Steps to reproduce

After re-enabling the subtheme for testing (which I had uninstalled), the options re-appeared. NTF!

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes