How to remove the blue background of button on mobile?

You can add:

-webkit-tap-highlight-color: transparent;

You can also add this to your stylesheets to define it globally:

input,
textarea,
button,
select,
a {
  -webkit-tap-highlight-color: rgba(0,0,0,0);
}

Hope this helps :)

You can find the documentation here for more info: https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/AdjustingtheTextSize/AdjustingtheTextSize.html#//apple_ref/doc/uid/TP40006510-SW5

* {
    -webkit-tap-highlight-color: transparent;
}

Test it.