[Docs] Typography in dark theme mode unreadable

[yzorg]

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

Expected Behavior

The Typography component demo should be usable in dark mode.

Current Behavior

Typography demo on docs site should be readable in dark mode. The background is hard-coded to white, but in dark mode the text color is changed to light gray, making the text nearly unreadable (only via gaps in the background ruler lines).

Steps to Reproduce (for bugs)

1. https://material-ui.com/style/typography/#component
2. find the toolbar (top right)
3. click light/dark lightbulb to change to dark theme

Context

Was planning to adopt the library for an internal site, using the dark theme. I was demonstrating to my boss and team, and they were a bit put off that some of the component demos didn't work in dark theme.

Your Environment

Tech	Version
Material-UI	1.0.0-beta.44
React	? docs site
browser	any
etc

Again

The same problem happens on the Theme page, Typography section.

1. browse to https://material-ui.com/customization/themes/#typography
2. click light/dark theme lightbulb in the top toolbar

[yzorg]

I have a proposed fix, demonstrated at https://codesandbox.io/s/8y9ywpp69j that uses the new Component-specific overrides support in theme. Would that be appropriate to use in this demo?