Use font in a custom theme - Lacro59/playnite-gameactivity-plugin GitHub Wiki

For use and see in theme editor the font from plugin, you must get the "font.ttf".

And copy in "Fonts" folder in your theme.

And you must reference it in Blend.

After, the font is called with "DynamicResource".
<TextBlock Name="PART_ButtonIcon" Text="" FontFamily="{DynamicResource CommonFont}" />