From this section you can control front-end view of WordPress Download Manager Pro:
The default UI library is Bootstrap 4. However, you may use Bootstrap 3 too based on your active theme.
Now, your theme may already have bootstrap CSS, js and font awesome integrated, in such case, you need to disable them from "Disable Style and Script" option.
More details are explained here:
For any technical issue, if you are already using pro version please post in pro forum and free version users please post in free forum. Otherwise, if you have any pre-sale or order related query please contact live chat support team. For technical support.