Load My Resources will load the animation frames of the cave animation, so FancyMenu can render them in menus.
Load My Resources is available on CurseForge:
FORGE: https://www.curseforge.com/minecraft/mc-mods/load-my-resources-forge
FABRIC: https://www.curseforge.com/minecraft/mc-mods/load-my-resources-fabric
Just copy the mod JAR file you've downloaded in the last step to your .minecraft/mods
folder.