How to change button colors in custom.css

Hi,

I have added a "Subscribe Module" at my site. I want to define a class in my custom.css to change the button colors to the standard template colors.
See arrow 1 : subscribe module. Arrow 2: Standard module of my template (Notio, Rockettheme)
How do I define such a class for the active button and the hover color? I have tried several things with firebug, but with no success :-(.

Thank you,

Kind regards, T.W. van Urk