Mobile Nav Menu Custom Font Issue

Hello! 
I just found a css code that works for changing the font on the mobile nav menu (links) to a custom font. It worked, but then when I close out it does a sudden flash of the old font. Does anyone know why it would be doing that or how I can make it not? I attached a photo of the css code I used and video of what happens. Thanks!

6 hours ago, tuanphan said:

Remove this ID from your code

This worked, thank you!