Like Celeste? Like Minecraft? Well this is a good place to start.
The Celestial Resort Resourcepack (yes that's the title) is a resourcepack that centers heavily on Celeste's collectibles, but is also a GUI retexture, with the Mirror Temple aesthetic if that's what you like.

This is a CIT (Custom Item Texture) resourcepack, so if you're only interested in having the GUI aspects of this resourcepack, Optifine isn't required, but if you do want the collectibles to appear, any 1.16 Optifine version should work just fine.

Here's the naming guide!

https://drive.google.com/file/d/1Y0w5VsLXkwqUO_SKplsivua1DJ40trzM/view

Reach out on Twitter (@Edg_Void) if you feel like I missed something!