How to fix a constant color of the font to white in the button "Ask a question"?

I am creating a site. When I hover the mouse on the submit button the color of the font changes to white. Before that hovering, the font color is greenish. I want to keep the color of the font to white all the time. It would be nice if  you please let me know where to change the code?
Best regards,
Khaled Reza