Update useThemeContext for changes in Canary.

This fixes issue #116 by applying the fix for the breaking change in the latest canary build.

Author: homotechsual

packages/docusaurus-theme-redoc/src/theme/Redoc/Redoc.tsx

@@ -1,6 +1,6 @@
 import React, { useMemo } from 'react';
 import { usePluginData } from '@docusaurus/useGlobalData';
-import useThemeContext from '@theme/hooks/useThemeContext';
+import { useColorMode } from '@docusaurus/theme-common';
 import { Redoc as RedocComponent, RedocStandalone, AppStore } from 'redoc';
 import { RedocProps as Props, GlobalData } from '../../types/common';
 import './styles.css';
@@ -12,7 +12,7 @@ import './styles.css';
  * Released under the MIT License
  */
 function Redoc(props: Props): JSX.Element {
-  const { isDarkTheme } = useThemeContext();
+  const { isDarkTheme } = useColorMode();
   const { lightTheme, darkTheme, redocOptions } = usePluginData<GlobalData>(
     'docusaurus-theme-redoc',
   );