Bug 1512489 accidently added a dark backgroung to the date/time picker (while some of the text there remained black and was unreadable). This was later fixed by bug 1522779.
Would be nice if there was a dark themed date/time picker, if the user enables the dark theme.
See attachment 9039040 [details] and attachment 9039041 [details] for how this looked prior to bug 1522779.
This doesn't seem to have much to do with bug 1519547. This is a widget used by web pages, not a Firefox in-content page.