How to Change Font Colour of Submit Button

Hi,

I am trying to change font colour for the “submit” button but to no avail. I see you have an extension called “very simple custom style” but it seems to be just a zip file and not a plugin? I would like to know how to change the font colour of the submit button, and if you could go step-by-step that would be much appreciated!

https://www.ads-software.com/plugins/very-simple-contact-form/