The latest versions. not sure how far back it started though.
The new YouTube dark theme isn't active yet and I'm not used to changing things in there on Substratum. I stick to colors.xml generally.
In Play Store I've checked the other values folders and found nothing. May be the color is hard-coded in the smali which I have no experience with whatsoever. In the mean time I've changed the main background of the app to be red to match the rest of my theme so that the text can be seen in black/grey over it.
I just got the new one but settings still themed. I've only added new colors about a month ago. So no idea. Can't be hard coded. That's a subs theme