Here's the high contrast mode in question:

I tried taking the texture to put in my custom-made resource pack (with a lot of fixes and features I like) to see if it would replace the texture but it doesn't work either.

It first appeared at some point when I updated the mod and other mods about 2 weeks ago. Could maybe another mod SOMEHOW be affecting how the Chest Tracker mod displays?

I thought it was one of my many resource packs at first but I never installed a resource pack that would do that and I tried placing another resource pack containing the textures of the mod (in the right places) on top but nothing happened.

Help

They're stored under 'high_contrast' in the resources root and loaded by Fabric API's resource loader. If you want to replace them, you should be able to do it by copying their paths.

https://github.com/JackFred2/ChestTracker/tree/1.21/src%2Fclient%2Fresources%2Fhigh_contrast