Change contact form button style

Hi there,

I am looking to change the style of the button used on the Boldial theme.

The contact form being used is the one you support, Contact Form 7.

The image below shows a grey button which is bland and boring (I know the colour can be changed, etc via css individually).

How can I change the code of the child theme so that all buttons by default are shown as the blue button in the image, a large button.