Form in widget: border CSS

Hi guys, On my website I have a link to my Paypal donation page. The style for this button is 'form'. You can see there is a border. I want to remove this border, but I can't figure out how. I think I need a CSS code to do this, but I don't know which one. I hope you can help me out. Thanx in advance. Website: http://www.sebastiaanhorn.nl