Thanks for posting! It looks like there’s a bit of a conflict between that CSS code you referenced and the pre-existing one on your WordPress theme. In testing a bit, you should be able to get the desired effect by changing “background-color” (in your referenced code) to just “background”.
In your WordPress theme, the current button style that it’s honoring is using the “background” attribute, so you’ll need to use that in your referenced code as well. With this change, your code should look something like this:
In the above code, you can set the button’s text color by changing the “color” line. Simply change #000 to the hex code of your choice, and it should change it from black to whatever color you’d like (for white, color: #FFFFFF;, will do!).