Add TS type definition for useTheme from emotion-theming (#1509)

Author: XeeD

.changeset/dirty-owls-remain/changes.json

@@ -0,0 +1,4 @@
+{
+  "releases": [{ "name": "emotion-theming", "type": "patch" }],
+  "dependents": []
+}

.changeset/dirty-owls-remain/changes.md

@@ -0,0 +1 @@
+Add TypeScript type definition for the useTheme hook in emotion-theming

packages/emotion-theming/types/index.d.ts

@@ -14,6 +14,8 @@ export function ThemeProvider<Theme>(
   props: ThemeProviderProps<Theme>
 ): React.ReactElement
 
+export function useTheme<Theme>(): Theme
+
 /**
  * @todo Add more constraint to C so that
  * this function only accepts components with theme props.

packages/emotion-theming/types/tests.tsx

@@ -1,7 +1,7 @@
 import * as emotionTheming from 'emotion-theming'
 import * as React from 'react'
 
-const { ThemeProvider, withTheme } = emotionTheming
+const { ThemeProvider, withTheme, useTheme } = emotionTheming
 
 interface Theme {
   primary: string
@@ -38,6 +38,11 @@ class CompCWithDefault extends React.Component<Props> {
   }
 }
 
+{
+  const theme: Theme = useTheme<Theme>()
+  const themeFail: Theme = useTheme<number>() // $ExpectError
+}
+
 const ThemedSFCWithDefault = withTheme(CompSFCWithDefault)
 ;<ThemedSFCWithDefault />
 ;<ThemedSFCWithDefault theme={theme} />