Missing font face typings

[trusktr]

• This is not a v0.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior 🤔

Using TypeScript, we should be able to make an array of font faces in a @global @font-face block:

    '@global': {
      '@font-face': [
        {fontFamily: '...', src: '...'},
        {fontFamily: '...', src: '...'}
      ]
    },

but it gives an error:

Current Behavior 😯

If I supply one font family instead of an array of font families, it works fine:

Steps to Reproduce 🕹

Link: https://codesandbox.io/embed/createreactappwithtypescript-zg47i

1. look at src/pages/index.tsx in the styles.

Context 🔦

I'd like to apply multiple font faces at once, using the standard JSS format (the @font-face can be an array).

Your Environment 🌎

Tech	Version
Material-UI	v3.8.3

[trusktr]

As a workaround, I can cast it:

    '@global': {
      '@font-face': [
        {fontFamily: '...', src: '...'},
        {fontFamily: '...', src: '...'}
      ] as unknown as CSSProperties
    },

[trusktr]

related discussion: DefinitelyTyped/DefinitelyTyped#35614