I can't quite see the screenshots and that URL is dead.

What display errors exactly for the buttons could you post the screenshots here?
 
Sorry, hopefully this helps.

http://cbr.bike/threads/mobile-theme.17/

IMG_0349.PNG
 
This would be the browser blocking font awesome. More than likely they're not seeing any other font awesome calls either throughout the style. Do they have an ad blocker enabled on the browser?
 
Is there any alternative/work around?

Some folks are referencing adblocking at the network level that they cannot disable (ie: using WiFi).

You can try changing the font awesome call:

Style Properties -> [XB] Font Awesome, switch it from from the CDN to local or vice versa.
 

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