Center the Contact-form widget button & its text vertically

Hi !
working with most of your fantastic widgets (yeah, sincerely, they’re great) I still have trouble when I want to :
– center the button of the contact form ?
can you please give me a tip ?
– also how can I center vertically the text of the button ?
Here is my link : http://www.severineautin.com/ceramics/

Many thanks ;)
Séverine