I am using EP lite on a clients account and recently switched from Twenty 10 to Twenty 12 theme which produces invisible buttons.
The EPL buttons become white text on a linear gray background. So while you can change the CSS as you recommend here
It doesn't actually work because there are further CSS styles set for the background image below e.g.
background-image: -moz-linear-gradient(top, #f4f4f4, #e6e6e6);
There are also similar background-image styles set for hover and active.
If you comment out all of the background-image declarations in the Theme style.css the buttons then follow the background style setting.
Hope this helps