There's nowhere we rely on this behavior (with a non-white background color), but it can make it harder for on-wiki users (@Jackmcbarn mentioned this to me).
Before, we were setting color but not background-color (setting one but not the other is always wrong, since it can conflict with user or system styles). In @matmarex's commit that fixed this (https://gerrit.wikimedia.org/r/#/c/168261/) he mentioned we might want to just remove it. The alternative is to set them to something reasonable like black (or dark grey) text and white background.
See T74192: MediaWiki UI: Unreadable text with dark system settings in .mw-ui-input