All the elements in the form, including the form itself have a specific class.
The form has the CSS class wpcf7-form. If you use Chrome’s developer tools ‘Inspect Element’ feature, or similarly Firefox Firebug you would be able to see each element in your form and what CSS class it has (starts with wpcf7-). In your theme’s CSS file you can then change borders, backgrounds, fonts or whatever you want.
The actual look of CF7 forms on your website will depend largely on the current WordPress theme used and the CSS styling that theme applies to standard HTML form elements.
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.
