## Core changes

JS is the primary source of theme variables for Grafana. Theme definitions are located in `@grafana/ui/themes` directory.

#### Themes are implemented in pure js.

This is because our goal is to share variables between app and SASS. To achieve that themes are necessary during build time to be exposed to sass loader via `node-sass` functions (https://github.com/sass/node-sass/blob/master/README.md#functions--v300---experimental). This retrieval is implemented in `getThemeVariable(variablePath, themeName)`.

#### Themes are available to React components via `ThemeContext`

Context is available via `import { ThemeContext } from '@grafana/ui';`

**If you skip `themeName` param, then dark theme's variant will be used**

## Using themes in Grafana

### SASS

`getThemeVariable` is a function, that's available in sass files. Use it i.e. like this:

```scss
// In theme agnostic SASS file
.foo {
  font-size: getThemeVariable('typography.size.m');
}

// In *.[themeName].scss
.bar {
  background-color: getThemeVariable('colors.blueLight', '[themeName]');
}
```

### React

#### Using `ThemeContext` directly

```ts
import { ThemeContext } from '@grafana/ui';

<ThemeContext.Consumer>{theme => <Foo theme={theme} />}</ThemeContext.Consumer>;
```

#### Using `withTheme` HOC

With this method your component will be automatically wrapped in `ThemeContext.Consumer` and provided with current theme via `theme` prop. Component used with `withTheme` must implement `Themeable` interface.

```ts
import  { ThemeContext, Themeable } from '@grafana/ui';

interface FooProps extends Themeable {}

const Foo: React.FunctionComponent<FooProps> = () => ...

export default withTheme(Foo);
```

### Storybook

All stories are wrapped with `ThemeContext.Provider` using global decorator. To render `Themeable` component that's not wrapped by `withTheme` HOC you either create a new component in your story:

```tsx
// Foo.story.tsx
const FooWithTheme = withTheme(Foo);

FooStories.add('Story' () => {
  return <FooWithTheme />
});
```

or use `renderComponentWithTheme` helper:

```tsx
// Bar.story.tsx

BarStories.add('Story' () => {
  return renderComponentWithTheme(Bar, /* pass props here */)
});
```

### Angular

There should be very few cases when theme would be used in Angular context. For this purpise there is a function available that retrieves current theme: `import { getCurrentTheme } from app/core/utils/ConfigProvider`

## Limitations

- #### Hot updates
  Changes in JS theme files _are not subject of hot updates_ during development. This applies to styles that comes from SASS files (which means 100% until we introduce css in js approach). This is a consequence of the fact that `getThemeVariable` util is executed during webpack pipeline.
- #### You must ensure ThemeContext provider is available in a React tree
  By default all react2angular directives have `ThemeContext.Provider` ensured. But, there are cases where we create another React tree via `ReactDOM.render`. This happens in case of graph legend rendering and `ReactContainer` directive. In such cases theme consumption will fail. To make sure theme context is available in such cases, you need to wrap your rendered component with ThemeContext.Provider using `provideTheme` function:

```typescript
// graph.ts
import { provideTheme } from 'app/core/utils/ConfigProvider';

// Create component with ThemeContext.Provider first.
// Otherwise React will create new components every time it renders!
const LegendWithThemeProvider = provideTheme(Legend);

const legendReactElem = React.createElement(LegendWithThemeProvider, legendProps);
ReactDOM.render(legendReactElem, this.legendElem, () => this.renderPanel());
```

`provideTheme` makes current theme available via ThemeContext by checking if user has `lightTheme` set in her boot data.