I was wondering if we could apply custom classes (bootstrap) to the submit button in WFFM? I know that form inputs seem to have this ability, but I don't seem to see this feature for the submit button itself.
You can create a custom style and assign it to each form individually.
Create an item under
/sitecore/system/Modules/Web Forms for Marketers/Settings/Meta data/Submit Button Type of template type
/sitecore/templates/Web Forms for Marketers/Meta Data/Extended List Item and set the Value to your custom CSS class.
Then on the form itself, you can select the style in the Submit Button type field. You have to do this from the Content Editor, there is no option to set this from the Form Designer.
The style on your submit button will be set as
btn custom-style-value. The value from Submit Button size field is also applied in the same way, so it's possible to use that for styling also.