I made a custom library for myself which works fine, but it's generated a *.ecss file which none of the other libraries have. Out of curiousity, can anyone tell me what it's for? It's purpose doesn't seem to be mentioned anywhere.
Try renaming it (as a txt file) and see if you can open it in Notepad for clues. If the library still works with the renamed file, it's probably safe to delete it.