After the recent OOjs UI design updates, mw.widgets.DateInputWidget no longer matches the design of other widgets in MediaWiki theme. None of the colors match and it really stands out among other widgets. Screenshot from UploadWizard:
I think it was meant to resemble DropdownInputWidget when not active, and TextInputWidget when active (it also has a dropdown, not shown here, because it has never matched anything much anyway – let's consider it out of scope for now).