CSS for submit button

You can modify the CSS of just about anything in the plugin by adding targeted CSS commands to your theme's style sheet. You should not directly modify native plugin files, as you changes will be lost when you need to update the plugin. There is an extensive list of CSS Targeting Samples in the plugin documentation including the single-page form footer and the multi-page form footer, which contain the submit or next/previous buttons, depending on whether your form has one or multiple pages. Hope that helps.