Button colors not working

Hello, the button colors and text are not changing properly. If I change the background color to a lighter color, I need for the text to be darker. For example #000000, however the text on the button changes to #000000 but my button background color is #000000. I have tried all sorts of variations of CSS to get this working. While it works in the browser, when that code is placed in the Additional CSS box, I clear all cache and the changes do not apply. Even if I clear the colors in the plugin settings, that doesn’t have any affect. How can we change the text while leaving the button text a different color?