By default, CSS styles controlling the appearance of form fields reside in the following dotCMS directory: /html/css/content/forms.css. However, redefining the CSS classes controlling the styling of form fields is fairly simple.
Setting the $contentFormMacroCSS velocity variable on the Template, Container, or in a piece of content above the form on the page will over-ride the default CSS controlling form field styling. An example of the set command to a relative path is given below:
#set($contentFormMacroCSS = "/global/css/mycustomcssfile.css")
As long as the customized CSS file is read after the default dotCMS file, and before the Form itself, then the styles for the form fields will be over-ridden.