Form submission button on mobile not behaving as it should

Hi 

I have a form on a client's site that is doing something strange. When the submit button of the form is clicked the button font color changes to red making it the same color as the button background and therefore invisible. It then shrinks a little before moving to the redirect page. I've looked at all my css and can't get to the bottom of this.

Strangely in mobile view on the backend it behaves as expected - red text white background but on an actual mobile device I get the issue as shown in video below.

Upon further testing It's not just the menu submission button but also the navigation call to action button on mobile too - it loses the text. So I'm guessing it's a button issue on the whole or theme issue?

The page with the offending form button
https://www.alfonsos.com.au/request-a-quote

Home page for mobile navigation menu (REQUEST A QUOTE)
https://www.alfonsos.com.au/

Thanks in advance, 
Helen

@tuanphan thank you so much - as always you come to my rescue - I'm just confused as to why we needed to add this code - is it to over-ride some CSS in the site? I would not have been able to find the answer to this as I can't see the cause.

I dont see an option to control button hover text, so I guess we need to use CSS code