There are just too many too choose from.
I was going to list'em all here but will refer you to this article, Best UI Frameworks for your new React.js App. instead.
There is one not mentioned there, which is Atlaskit created by Atlassian (best known for JIRA).
I've never used Atlaskit but it seems not too overbearing or too flat like other Material based components.
I've used Ant-Design and wasn't so bad to work with.
And lastly, there is a React framework called React-admin (Material based), which lets you build administrative pages.
What you need to be aware of when using those components is that,
For themes, checkout Creative Tim (sorry, never used it but only heard of it here and there.).
Thank you, whats your thoughts on github.com/ipselon/structor structor
Wow, it looks cool.
I probably won't be able to give you a feedback until I play around with it, though...
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