Hmm, it’s hard to tell what other styles are affecting your theme. I’d recommend taking a look at this guide if you haven’t already, particular the parts that show using the browser’s inspector (in the Dev Tools – accessed with F12 in Chrome). It lets you see what styles of being applied to each element and toggle them on and off to see the difference in realtime.
Since it’s related to the mobile view, you can emulate mobile on your desktop from most browsers. Here’s what the button looks like in Chrome:
