I replied to your question in the support ticket you opened, but in case anyone else is looking for the same information, I will copy the explanation from the ticket here as well.
Adding a custom font in an AppBuilder mobile app happens the same way as you would add it to a regular web app. One article that elaborates on this in details is available here:
Setting Weights And Styles With The @font-face Declaration
I am also attaching a sample app where I applied the style linking approach described in the article.
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