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. If you are not familiar with CSS, this page also includes some links to where you can learn CSS.
The most common and preferred method is to create a Child Theme and make any necessary additions to the CSS in the child themes style.css only, rather than directly in the themes styles.css. That way you don't loose your changes when you update the theme.
It's also a good idea to use Firebug or Chrome Dev Tools to find and target individual CSS classes & ids within your CF7 forms, so that your CSS changes don't inadvertantly effect other elements on your website.
If you are uncomfortable editing CSS style sheets you may prefer to use a custom CSS plugin like My Custom CSS.