The enclosed EDG.css file does NOT need to be in the same folder as the enclosed HTML file. It may be placed in that folder, or in any folder up to ten levels "higher" (closer to the root). Your main e-books folder is probably a good choice.
The first LINK element in the HTML file will try to load EDG.css from a folder ten levels above the folder that contains the HTML file. If the root folder isn't ten level above the HTML file, the LINK element does nothing.
The following LINK elements will do the same, but from folders that are farther and farther "down" from the root, closer to the the folder that contains the HTML file. Finally, the last LINK element will try to load EDG.css from the folder that contains the HTML file.
Any copy of EDG.css that is loaded later, such as one in the same folder as the HTML file, will be able to override CSS instructions from any copies that were loaded earlier, such as a copy in your main e-book folder. This arrangement allows a single, possibly customized, copy of EDG.css to provide CSS instructions for many HTML files, and for special customizations on a per-folder-tree basis.
If you want, you can have different, customized versions of EDG.css for the different folders of books. Simply place your customized version of EDG.css into those folders and all HTML files in those folders (the nested folders) that use the LINK elements as in the enclosed HTML file will use your customized version of EDG.css.
If you want to update an HTML file to use this method of loading an EDG.css file (or files), insert the following block of LINK elements immediately before the tag in your HTML file. Do NOT include the "======" lines.