Several textures for slots that indicate what items can be placed inside them include a color or the entire slot texture as a background instead of having a transparent background like the armor slot textures for the player inventory. These textures include (all are located in assets/minecraft/textures/gui/sprites/container):
horse/armor_slot.png
horse/llama_armor_slot.png
horse/saddle_slot.png
loom/banner_slot.png
loom/dye_slot.png
loom/pattern_slot.png
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This is a feature request

I would strongly recommend having this be looked at again. 24w39a has made changes to how the slot highlight effect appears (most of it is rendered under the item, and only a slight amount on top). This has resulted in MC-277202 arising. In order for that to be fully fixed, the gray background will need removed from these texture files.
Can confirm in 23w33a. Here's some fixed loom textures:
[media]For horses and llamas, removing the "background" removes the entire slot from the UI, so code changes will be required to fix this.