Changing the form button colour

I am evaluating the plugin for a client and it looks like the ideal plug-in with the additional custom fields add-on and should meet their requirements. However, I am not able to see any way to change the colour of the button on the initial product page, there is guidelines on the web site for the Stripe Pay button.

I am assuming that this can be done with CSS – and would be grateful if you could advise how I change the background, text and indeed button size so that it can blend in with their branding colours.