The date/time input was particularly disappointing... the time being absolutely hideous even on the desktop. These days I consider material-ui.com (react components) to be one of the best component libraries I've seen. Short of that, I'd just assume use something like Bootstrap for the (s)css and control most of it myself, which is easy enough.

In fact, I'm half surprised they didn't just do a nice set of components meant to be used with Bootstrap, considering the style choices are very close.