How are you changing the font, specifically? Try modifying the Normal style. Press Ctrl+Shift+S to display the Apply Styles pane. Type in Normal and click the Modify button. Make the desired changes.

This is going to work assuming that your custom styles are based on Normal. If they are not, you have to explicitly modify the font of each affected style in your document.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional