See http://blog.teamtreehouse.com/the-css3-placeholder-pseudo-element – that is best explanation I can find on how to style placeholder text.
Use Firebug or Chrome Dev Tools to to examine the HTML and CSS of your CF7 form in detail. Once you understand how the HTML and CSS is configured in your CF7 form, you should be able to see the CSS changes you need to change the appearance of the form to suit your requirements.
Add your CSS via Child Theme or use custom CSS plugin.
Also 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.
