Shadow issue with resource pack

Hello, i have an issue with the shadows when i put on my texture pack. I don't know why everything looks so dark.

I've heard i should add a file in my texture pack to fix that, could anyone help me out with that please?

i'm using Iris 1.6.17, with last version of rethinking voxel and Excalibur's newest version as resource pack

Thanks for your help!