Thank you so much for letting me use it! Also, I have an idea. I made a new version of the mod, and although the new version isn't compatible with optifine : (, it has the mod use a new file called custom_end_sky.png. Basically when the file is in the correct place, it will load the custom sky, but when it isn't, it will load the normal end sky. This means that it would be possible to make a resource pack that has a custom sky when being loaded with the mod, but has no effect without the mod!
(with the previous version of the mod, the resource pack had to be modified in a way that without the mod, the end sky would look like a bunch of repeating squares of the png file.)

At the moment, there aren't any resource packs that have the custom_end_sky.png file. If there were though, it would be much more convenient for people who download my mod because all they would have to do is download the resource pack without having to modify it! For people not using the mod, the only difference would be a slightly larger file size.
Would you like to change your resource pack to make it more convenient for the people using my mod? In like 3 days I already got like 75 downloads which is insane! If not, it's completely fine. I was gonna ask how to make custom skies like yours, but then I got distracted, googled it on youtube, and already found 2 tutorials, so I could probably make one, it would just be great if yours could work.