I've noticed that changing the setting from "use custom dynamic forms style sheet" to "use standard dotnetnuke style sheet" doesn't really work. It looks like the html always points to the style sheet (even if you have a fresh module that was never changed to use a custom stylesheet, the html still points to a css file in your portal root, that doesn't exist). The only way to truly switch back to a "standard DNN style sheet" is to delete the css file. It's ok if the stylesheet is not deleted when this option is changed, but then the html/css code should be removed or commented out of the page (on render).
Also, it would be great if the stylesheets & js files were created in a sub directory and not the portal root. Even better might be to create a core "dynamic forms" directory with sub-directories that would include js, css, file exports, pdf's, etc.