[bug] isDarkTheme from useThemeContext always returns false

[apsdehal]

🐛 Bug Report

In the default template for docusaurus, I am trying to use theme context to get isDarkTheme's value. It is always set to false, even when I change to dark theme and then reload.

Have you read the Contributing Guidelines on issues?

Yes

To Reproduce

(Write your steps here:)

1. Use `const {isDarkTheme} = useThemeContext()
2. add console.log for isDarkTheme
3. turn on dark theme and reload

Expected behavior

Should provide true when dark theme is set

Actual Behavior

isDarkTheme is false even when dark them is on

Your Environment

• Docusaurus version used:2.0.0-alpha.56
• Environment name and version (e.g. Chrome 78.0.3904.108, Node.js 10.17.0): Node version: v12.17.0
• Operating system and version (desktop or mobile): Ubuntu 16.04

[SamChou19815]

See #2646. It might be caused by the same issue.

[apsdehal]

@SamChou19815 Thanks a lot. This does fix the issue. Maybe it needs to be documented better.

[slorber]

I suggest we move to a non-default themeContext value + a clear error message to tell the user what he did wrong. Could link to an issue.

[JoelMarcey]

@apsdehal Willing to submit a PR with better docs? 😊

[slorber]

docs has been updated here by @Drewbi : #2934

still think it would be cool if the code did actually warn the user in case of using the hook without a provider (could throw with a link to the docs)