To change the style of your CF7 forms you would need to edit the CSS style sheets used by your WordPress theme.
See Styling Contact Form for a general explanation of styling CF7 forms using CSS.
There is a link at the bottom of the page to a comprehensive and detailed article on Styling Contact Form 7 Forms. The article shows people, with suitable HTML & CSS skills, how to change the appearance of their Contact Form 7 Forms to meet their particular requirements.
If you find you need more detailed advice, after reading and working through the above links, please include a link to your Contact Form 7 form, so others here can examine your form in detail using Firebug or Chrome Dev Tools to understand the CSS used for your CF7 form elements and provide you with a possible solution.
