Theme example breaks with AppBar usage

[matthewoates]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

https://codesandbox.io/s/material-demo-qjiv5

This breaks with the suggested theme object:

const themeInstance = {
  background: "linear-gradient(45deg, #FE6B8B 30%, #FF8E53 90%)"
};

To fix the cannot set property type of undefined error:

const themeInstance = {
  background: "linear-gradient(45deg, #FE6B8B 30%, #FF8E53 90%)",
  palette: {
    type: "dark"
  }
};

And to fix that error:

import { grey } from '@material-ui/core/colors';

const themeInstance = {
  background: "linear-gradient(45deg, #FE6B8B 30%, #FF8E53 90%)",
  palette: {
    type: "dark",
    grey
  }
};

and I'm presented with yet another error. I'd expect the theme manager to merge the theme object correctly so that fields aren't missing.

[matthewoates]

It looks like the docs here:

• https://material-ui.com/styles/advanced/

and the Codesandboxes need to be updated to include createMuiTheme({ background: ... })

[eps1lon]

Yeah the example is not very helpful since it uses /core but wouldn't work with /core components. We should wrap it in createMuiTheme.

[oliviertassinari]

Definitely, it will be clearer with something like this:

diff --git a/docs/src/pages/styles/advanced/advanced.md b/docs/src/pages/styles/advanced/advanced.md
index 53227692a..c192f2c1a 100644
--- a/docs/src/pages/styles/advanced/advanced.md
+++ b/docs/src/pages/styles/advanced/advanced.md
@@ -6,7 +6,7 @@

 Add a `ThemeProvider` to the top level of your app to pass a theme down the React component tree. Then, you can access the theme object in style functions.

-> This example creates a new theme. See the [theming section](/customization/theming/) for how to customize the default Material-UI theme.
+> This example creates a minimalist theme object. Head to the [theming section](/customization/theming/) if you intend to use some of the Material-UI's components. You need to provide a richer theme structure using the `createMuiTheme()` method.

 ```jsx
 import { ThemeProvider } from '@material-ui/core/styles';

@matthewoates do you want to submit a pull request? :)

[maksimgm]

PR created:
#20339