Add documentation on how to override Backstage components

Signed-off-by: Daniel Ortiz Lira <dortiz@vmware.com>

docs/getting-started/app-custom-theme.md

@@ -37,9 +37,7 @@ exported by [@backstage/theme](https://www.npmjs.com/package/@backstage/theme)
 in combination with
 [createTheme](https://material-ui.com/customization/theming/#createmuitheme-options-args-theme)
 from [@material-ui/core](https://www.npmjs.com/package/@material-ui/core). See
-the
-[@backstage/theme source](https://github.com/backstage/backstage/tree/master/packages/theme/src)
-and the implementation of the `createTheme` function for how this is done.
+the "Overriding Backstage and Material UI css rules" section below.
 
 You can also create a theme from scratch that matches the `BackstageTheme` type
 exported by [@backstage/theme](https://www.npmjs.com/package/@backstage/theme).
@@ -136,6 +134,130 @@ const themeOptions = createThemeOptions({
 });
 ```
 
+## Overriding Backstage and Material UI components styles
+
+When creating a custom theme you would be applying different values to
+component's css rules that use the theme object. For example, a Backstage
+component's styles might look like this:
+
+```ts
+const useStyles = makeStyles<BackstageTheme>(
+  theme => ({
+    header: {
+      padding: theme.spacing(3),
+      boxShadow: '0 0 8px 3px rgba(20, 20, 20, 0.3)',
+      backgroundImage: theme.page.backgroundImage,
+    },
+  }),
+  { name: 'BackstageHeader' },
+);
+```
+
+Notice how the `padding` is getting its value from `theme.spacing`, that means
+that setting a value for spacing in your custom theme would affect this
+component padding property and the same goes for `backgroundImage` which uses
+`theme.page.backgroundImage`. However, the `boxShadow` property doesn't
+reference any value from the theme, that means that creating a custom theme
+wouldn't be enough to alter the `box-shadow` property or to add css rules that
+aren't already defined like a margin. For these cases you should also create an
+override.
+
+_Creating your custom overrides_
+
+```ts
+import {
+  BackstageTheme,
+  createThemeOverrides as createBackstageOverrides,
+} from '@backstage/theme';
+
+/**
+ * The `@backstage/core-components` package exposes this type that
+ * contains all Backstage and `material-ui` components that can be
+ * overridden along with the classes key those components use.
+ */
+import { BackstageOverrides } from '@backstage/core-components';
+
+export const createCustomThemeOverrides = (
+  _: BackstageTheme,
+): BackstageOverrides => {
+  return {
+    /**
+     * You can reference the component by the name it passed to the makeStyles function.
+     * in the previous example it was { name: 'BackstageHeader' }
+     */
+    BackstageHeader: {
+      header: {
+        margin: '20px',
+        boxShadow: 'none',
+      },
+    },
+  };
+};
+
+export const createOverrides = (theme: BackstageTheme): BackstageOverrides => ({
+  ...createBackstageOverrides(theme), // These are the overrides that Backstage applies to `material-ui` components
+  ...createCustomThemeOverrides(theme), // These are your custom overrides, either to `material-ui` components or to Backstage components.
+});
+```
+
+_Your custom theme that uses the overrides_
+
+```ts
+import { createTheme as createMuiTheme } from '@material-ui/core';
+import { AppTheme } from '@backstage/core-plugin-api';
+import {
+  BackstagePaletteOptions,
+  BackstageTheme,
+  BackstageThemeOptions,
+  PageTheme,
+  shapes,
+} from '@backstage/theme';
+
+import { createOverrides } from './customThemeOverrides';
+import { THEME_OPTIONS } from './themeOptions'; // This is the object created in the "Example of a custom theme" section
+
+export const createTheme = (
+  paletteOptions: BackstagePaletteOptions,
+): BackstageTheme => {
+  const pageTheme: PageTheme = {
+    colors: [],
+    shape: shapes.round,
+    backgroundImage: '',
+  };
+
+  const themeOptions: BackstageThemeOptions = {
+    palette: paletteOptions,
+    page: pageTheme,
+    getPageTheme: _ => pageTheme,
+  };
+
+  const baseTheme = createMuiTheme(themeOptions) as BackstageTheme;
+  const overrides = createOverrides(baseTheme);
+
+  return { ...baseTheme, overrides } as BackstageTheme;
+};
+
+export const CustomTheme: AppTheme = {
+  id: 'custom-theme',
+  title: 'Custom Theme',
+  variant: 'light',
+  theme: createTheme(THEME_OPTIONS),
+};
+```
+
+_Aplying your custom theme with overrides_
+
+```ts
+import { createApp } from '@backstage/core-app-api';
+import { CustomTheme } from './theme/customTheme';
+
+const app = createApp({
+  apis,
+  themes: [CustomTheme],
+  plugins: Object.values(plugins),
+  ...
+```
+
 ## Custom Logo
 
 In addition to a custom theme, you can also customize the logo displayed at the