Limit theme slug renaming to just slugs

[pbking]

When exporting the theme without applying translations or asset localization, the name of existing theme assets are changed.

• Clone the theme "Adventurer"
• Note that one of the assets used has the string "adventurer" in it. The reference to that asset in the patterns is changed to the NEW slug, though the file is not changed.

[pbking]

Alternatively, since the issue at hand was that assets used in the theme that have the slug in the filename have been changed, we could consider changing the file to contain the new slug instead.

That feels more like a bandaid, but at least doesn't result in broken image assets for those themes that are effected.