This is Trash Slot mod:

This is Filters mod:

And this is World Handler mod:

As the slot is a custom graphic (just like many other mod GUIs, such as the background of the World Handler mod you showed), it will only match the texture pack if the texture pack (or an addon for it) comes with a replacement graphic for it.

Yup

Thanks :(

So it's up to the texturepack artist to address it right?