there is no dark UI for horses and donkeys and mules and it's weird

You have to update the texture yourself.
How to do that is described here: https://www.curseforge.com/minecraft/mc-mods/more-banner-features/screenshots

for those who want the mod with dark UI

here is the link(unzip):
morebannerfeatures-1.1.3-1.18_darkui.zip