Open wp-contact-form-7.php file from the Contact Form 7 directory (under plugins) in your HTML editor. Search for input type="text". Add class="mybg" so the line looks like
$html = '<input type="text" class="mybg" name=...
Now search for
textarea. Once you find it add the class attribute so the line looks like this:
$html = '<textarea class="mybg" name=...
Save the file and upload it.
Now open stylesheet.css file (from the same directory) and add the following code to the end of the file:
Of course change the #e3d7b5 to whatever color you want.
Hope this helps.