To fix this you will need to modify the CSS of the relevant html form elements in your CF7 form.
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.
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.