docs(v2): more examples on lifecycle apis, cleanup (#2051)

* docs(v2): more examples on lifecycle apis, cleanup

* typo

website/docs/using-themes.md

@@ -1,7 +1,6 @@
 ---
 id: using-themes
 title: Themes
-sidebar_label: Introduction
 ---
 
 Like plugins, themes are designed to add functionality to your Docusaurus site. As a good rule of thumb, themes are mostly focused on client-side, where plugins are more focused on server-side functionalities. Themes are also designed to be replace-able with other themes.
@@ -101,6 +100,37 @@ This will copy the current `<Footer />` component used by the theme to a `src/th
 - [@docusaurus/theme-search-algolia](https://github.com/facebook/docusaurus/tree/master/packages/docusaurus-theme-search-algolia)
 - [@docusaurus/theme-live-codeblock](https://github.com/facebook/docusaurus/tree/master/packages/docusaurus-theme-live-codeblock)
 
+## Themes design
+
+While themes share the exact same lifecycle methods with plugins, their implementations can look very different from those of plugins based on themes' designed objectives.
+
+Themes are designed to complete the build of your Docusaurus site and supply the components used by your site, plugins, and the themes themselves. So a typical theme implementation would look like a `src/index.js` file that hooks it up to the lifecycle methods. Most likely they would not use `loadContent`, which plugins would use. And it is typically accompanied by a `src/theme` directory full of components.
+
+To summarize:
+
+- Themes share the same lifecycle methods with Plugins
+- Themes are run after all existing Plugins
+- Themes exist to add component aliases by extending the webpack config
+
+## Writing customized Docusaurus themes
+
+A Docusaurus theme normally includes an `index.js` file where you hook up to the lifecycle methods, alongside with a `theme/` directory of components. A typical Docusaurus `theme` folder looks like this:
+
+```shell {5-7}
+website
+├── package.json
+└── src
+    ├── index.js
+    └── theme
+        ├── MyThemeComponent
+        └── AnotherThemeComponent.js
+```
+
+There are two lifecycle methods that are essential to theme implementation:
+
+- [getThemePath](lifecycle-apis.md#getthemepath)
+- [getClientModules](lifecycle-apis.md#getclientmodules)
+
 <!--
 
 Outline