As you said
The look of the email and download forms can be changed by copying files from the /css/ and /templates/ folder of the plugin into your active theme folder. Rename the files ssd-original_file_name.php/css (e.g. ssd-download.php or ssd-ss-downloads.css) and edit as needed.
I really do not understand. I usually make the same folder structure in my active theme and put the files I want to edit/modify, but it doesn't work in this plugin
Anyway, great work.