[TB] Lockable Attributes

The ability for theming attributes within a parent theme to be locked so that sub-themes based on the parent theme retain the parent styling for the locked attributes. This is beneficial for keeping sub-themes on-brand.

## Suggestion/Concern
Attributes cannot be locked within parent theme to be locked. This would be beneficial for keeping sub-themes on-brand.

## Proposed Solution
allow for locking of attributes within a parent theme so that sub-themes based on the parent theme retain the parent styling for the locked attributes

No files, tests, or entry points are named. Start by locating how parent themes, sub-themes, and theme attributes are represented and inherited, then clarify the locking rules and identify tests that define inheritance behavior; done means locked parent attributes remain unchanged in sub-themes while other attributes remain customizable.

Written by the indexing model from the issue text.