In the qa-styles.css file find .qa-search, and below it add:
.qa-search-button {background:white; border: 1px solid black;}
This is just a simple example of adjusting the background color and border. You can also adjust the css on hover by adding:
.qa-search-button:hover {background:grey; border: 1px solid black;}
And of course this is fully customizable, just adjust colors and other css features as desired.