Custom fonts for Message section `Message Center & In-App Message

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I just want to know about how we can apply custom fonts for message center title and in app message content inside Message section. As font stack is not working for message section please check below image for the same.

• Message Center: I am able to customize the font only for message body through html but for Title not able to do the same.  
• In-App Message: For this also i am not able to apply the custom fonts.

So can you please let me know how I can do it. it's important to improve the In- App Experience.