How to Install and Enable the Structures Resource Pack for Loading Backgrounds
The Structures pack is a resource pack, not a mod, so it loads through the normal resource pack menu rather than the mods list. It only does anything if the Loading Backgrounds mod is already installed, since the mod is what reads the images and draws them behind the loading bar.
Placing the Pack
1. Open your resource packs folder from the in-game Options > Resource Packs screen, or drop the pack file into the resourcepacks folder inside your Minecraft instance directory.
2. Return to the game and open Options > Resource Packs.
3. Find the Structures pack in the left column (available packs) and move it to the right column (selected packs).
4. Click Done. The pack should now sit above or below your other packs depending on what you want it to override.
Confirming It Works
Leave the world and re-enter it, or switch between dimensions, so a loading screen appears. If you still see the mod's default backgrounds, check that the pack is actually enabled and that no other resource pack is overriding its files. Resource packs that change the same loading images will conflict, so keep only one background pack active at a time.
If nothing changes at all, verify the Loading Backgrounds mod itself is present and enabled for your instance.