What you can do is add your custom CSS classes for each of your form fields by editing the form in Sitefinity backend then edit the properties of the controls you want to style. On the appearance tab you can add custom CSS class as it shown in the attached screenshot. For example custoDescription, customFormSubmit and then apply the same CSS as Josh suggested:
/* sfFieldWrp is a Sitefinity class surrounding the input */
/* sfTxtLbl is a Sitefinity class applied to the label element */
I believe this should help you. If this does not help, please send more information about your Sitefinity version and how you are adding your custom CSS on the page.
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