message: add note about shallow merge
diff: @@ -78,7 +78,7 @@ This will set the `color`, `font-family`, and `line-height` CSS properties to th
  #### Theming
  -Components are styled using Primer's theme by default, but you can provide your own theme by using [styled-component's][styled-components] `<ThemeProvider>`. If you'd like to fully replace the Primer theme with your custom theme, pass your theme to the `<ThemeProvider>` in the root of your application like so:
  +Components are styled using Primer's [theme](https://github.com/primer/components/blob/master/src/theme.js) by default, but you can provide your own theme by using [styled-component's][styled-components] `<ThemeProvider>`. If you'd like to fully replace the Primer [theme](https://github.com/primer/components/blob/master/src/theme.js) with your custom theme, pass your theme to the `<ThemeProvider>` in the root of your application like so:
  ```jsx
  import {ThemeProvider} from 'styled-components'
  @@ -117,6 +117,8 @@ const App = (props) => {
  }
  ```
  +*Note that using `Object.assign` will only create a shallow merge. This means that if both themes have a `color` object, the _entire_ `color` object will be replaced with the new `color` object, instead of only replacing duplicate values from the original theme's color object.
  +
  #### Static CSS rendering
  If you're rendering React components both server-side _and_ client-side, we suggest following [styled-component's server-side rendering instructions](https://www.styled-components.com/docs/advanced#server-side-rendering) to avoid the flash of unstyled content for server-rendered components. This repo's [documentation template component](https://github.com/primer/components/blob/master/pages/_document.js) demonstrates how to do this in [Next.js].
  