Mango Buttons not appearing in Safari

https://www.tipsdaddy.com/sunderland-vs-leicester-predictions-betting-tips-and-preview/

For the url above, in Safari the button (generated by Mango Buttons plugin) which says BET NOW doesn't appear correctly whereas in Chrome it does.

It must be something to do with the CSS but I can't figure out how to fix it?

Thanks,

Rich