changing site theme doesn't make Tome regenerate assets

Problem/Motivation

I did a Tome export with Drush, and that worked fine.
Then I changed the site theme, and did another export.
That had HTML files showing broken CSS.
I then deleted the Tome output folder, and the next time I did a tome export, I saw this additional bit:

Processing related assets and paths...
 95/95 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

It looks like Tome is skipping processing of CSS and JS if it thinks they match what's already in the output folder. But this system didn't take into account the theme change.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes