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It's in one of the GFX files, which are difficult to mod unless one uses professional software. It's been discussed a bit back in 2014 or early 2015, and I think the actual GFX file with the white UI was identified. Search here in modding discussion and at the official forums in the Technical Modding Forum for GFX and Scaleform which is the program the Sims4 team uses IIRC.
MedievalMods and Sims3mods: Dive Cave Reset Fix, Resort Revamp, Industrial Oven Revamp, Will O' Wisp fix, UI Sounds Disabled, No Cars, Gnome Family Planner, Townies Out on the Town, No Martial Arts Clothes, Fast Skilling, etc.
http://simsasylum.com/tfm/