PowerPoint themes carry 12 colors; Storyline uses 16. The 11 shared colors map over directly. PowerPoint's 12th color, Followed Hyperlink, has no Storyline equivalent and is ignored.
Storyline's 5 extra roles — Control 1, Custom 1, Custom 2, Quiz Correct, and Quiz Incorrect — don't exist in a PowerPoint theme, so they're prefilled automatically on import. Review and adjust them before exporting.
If you add a Google Font, it's recommended to also install that same font family locally so it renders smoothly when the theme is opened in Storyline.