If you have applied a custom theme to a presentation and saved it the theme will be removed if you delete the theme from the hard drive.

Question

Quizplus · Accepted Answer

Once a custom theme is applied to a presentation and saved, the theme's elements (colors, fonts, effects) are embedded within the presentation itself. Deleting the theme file from the hard drive does not remove the theme from any presentation it has been applied to.

If You Have Applied a Custom Theme to a Presentation