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.
Most of the time, theme is used to provide a set of React components, e.g. `Navbar`, `Layout`, `Footer`.
Users can use these components in their code by importing them using the `@theme` webpack alias:
```js
import Navbar from '@theme/Navbar';
```
The alias `@theme` can refer to a few directories, in the following priority:
1. A user's `website/src/theme` directory, which is a special directory that has the higher precedence.
1. A Docusaurus theme packages's `theme` directory.
1. Fallback components provided by Docusaurus core (usually not needed).
Given the following structure
```
website
├── node_modules
│ └── docusaurus-theme
│ └── theme
│ └── Navbar.js
└── src
└── theme
└── Navbar.js
```
`website/src/theme/Navbar.js` takes precedence whenever `@theme/Navbar` is imported. This behavior is called component swizzling. In iOS, method swizzling is the process of changing the implementation of an existing selector (method). In the context of a website, component swizzling means providing an alternative component that takes precedence over the component provided by the theme.
**Themes are for providing UI components to present the content.** Most content plugins need to be paired with a theme in order to be actually useful. The UI is a separate layer from the data schema, so it makes it easy to swap out the themes for other designs (i.e., Bootstrap).
For example, a Docusaurus blog consists of a blog plugin and a blog theme.
Docusaurus Themes' components are designed to be replaceable. To make it easier for you, we created a command for you to replace theme components called `swizzle`.
This will copy the current `<Footer />` component used by the theme to a `src/theme/Footer` directory under the root of your site, which is where Docusaurus will look for swizzled components. Docusaurus will then use swizzled component in place of the original one from the theme.
Sometimes, you just want to wrap an existing theme component with additional logic, and it can be a pain to have to maintain an almost duplicate copy of the original theme component.
In such case, you should swizzle the component you want to wrap, but import the original theme component in your customized version to wrap it.
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
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:
