Luther MOBI exports have error in Part title pages

[mcgratay]

Description

When users export MOBI files on the Luther theme, Part titles read as: Part,"I
Problem occurs on Pressbooks.com and Textopress.

Exported MOBI on Textopress
- In Luther

• Errors in MOBI part titles: Part,”II

- In Bradbury Color

• No errors

- In Dillard

• No errors

Exported EPUB on Textopress
- In Luther

• No errors

Exported MOBI on Pressbooks.com
- In Luther

• Errors in MOBI part titles: Part,”II

Steps to Reproduce

1. Go to a book on the Luther theme (ex: https://luthertestkamron.textopress.com)
2. Make sure the book has more than one part and that Part and Chapter Numbers are set to display under Appearance > Theme Options > Global Options
3. Export the MOBI file
4. Open in Kindle Previewer

Expected Behavior:

Part titles read as Part I, Part II (et cetera)

Actual Behavior:

Part titles read as Part,"I, Part,"II (et cetera)