Hover colour spreading out beyond submit button

That’s happening because of css rule. Here’s how you can solve this:

1. Navigate to: wp-content/themes/wp-store/style.css

2. Go to line 452, you’ll see below code

   button:hover:after {
     opacity: 1;
     filter: alpha(opacity= 100);
     visibility: visible;
     width: 100%;
   }
   

3. Set the opacity to 0 OR remove this style completely.

NOTE:

1. I recommend you to create child theme, and make modifications there.
2. Please be aware, it will remove this styling from all other buttons.