Google's "Material Design" is becoming quite popular for styling web and mobile apps, almost like new (Twitter) Bootstrap library. Even Microsoft's Xamarin provides "Material" Templates.
There is also "Material Design Bootstrap" for jQuery, Angular, React, View.
For React there is also very popular:
Material-UI v4 is out 🎉 – Material-UI – Medium
Release v4.0.0 · mui-org/material-ui @ GitHub
"Some statistics with v4 compared to the release of v1 one year ago:
From 300k downloads/month to 2M downloads/month on npm
From 90k users/month to 350k users/month on the documentation"