If you've created your own theme from scratch, the fonts on the automated title page will be based on the font you selected for the Chapter Title in your theme settings. For pre-designed themes, the fonts used on the Title Page may also use Chapter Subtitle font for elements like the Book Subtitle or Author Name.
If you would like more control over the appearance of your title page, you have two options:
Replace the title page with a full-page image or a designed image of your book cover.
Delete the auto-generated title page and add a new chapter with your book or part details.