I have created a CRED Post Add form. In the form I have a category field set to "select". I've added to that field "class='categorySelect'. In the CSS section of the form I added some CSS to style ".categorySelect". However, after updating the form and previewing, the style name is not applied to that field.
You can either wrap your shortcode in a div and add the class to that to be able to target your select box, or use your browser dev tools to inspect the generated select box on the front end and identify an alternate CSS selector to target the select box directly.
For standard categories, something like this should work: