Material UI 4.x ignores bodyFontFamily

[clarmso]

A theme from typography.js works with Material UI 3.x, but the theme may not be applied once I upgraded to Material UI 4.x.

Here is a codesandbox on how to reproduce the bug:
https://codesandbox.io/s/gatsby-starter-haezl-zru2v

This Gatsby theme uses the fairyGatesTheme from typography.js, which has "Quattrocento Sans" as the body's font. The default body font for Material UI is Roboto. When material-ui/core 3.x is installed, the body's font is Quattrocento. When material-ui/core 4.x is installed, the body's font is Roboto.

I have not found a workaround other than sticking with Material UI 3.x. Please feel free to suggest that this issue may be related to gatsby-plugin-typography or material-ui/core instead.

[clarmso]

Maybe duplicate of #236