Letting WebView on Android work with prefers-color-scheme: dark

What I do, based on your question code, is to force based on current status:

int nightModeFlags = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
if (nightModeFlags == Configuration.UI_MODE_NIGHT_YES) {
  webSettings.setForceDark(WebSettings.FORCE_DARK_ON);
}

This way @media (prefers-color-scheme: dark) works, but @media (prefers-color-scheme: light) still doesn't work (tried using FORCE_DARK_OFF and FORCE_DARK_AUTO in an else)

Android Webview handles day/night mode a bit differently from the rest of the views. Setting your theme to dark will change the WebView components (scrollbar, zoom buttons etc.) to a dark mode version, but will not change the content it loaded.

To change the content you need to use the setForceDark method of the webview settings to make it change its contents as well. A compatibility version of this method can be found in the AndroidX webkit package.

Add the following dependency to your gradle build:

implementation 'androidx.webkit:webkit:1.3.0'

(1.3.0 is the minimum required version of this package. But higher versions should work as well.)

And add the following lines of code to your webview intitialization:

if(WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK)) {
    WebSettingsCompat.setForceDark(myWebView.getSettings(), WebSettingsCompat.FORCE_DARK_ON);
}

The isFeatureSupported check is there to make sure the Android System WebView version the user has installed on their device supports dark mode (since this can be updated or downgraded independently from the Android version through Google Play).

Note: The setForceDark feature requires Android System WebView v76 or up to be installed on the running device.

The force dark feature for webview content has two so-called strategies:

• User agent darkening: The webview will set its content to dark mode by automatically inverting or darkening colors of its content.

• Theme based darkening: The webview will change to dark mode according to the theme of the content (which includes the @media (prefers-color-scheme: dark) query).

To set which strategy the webview should use to apply force dark you can use the following code:

if(WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK_STRATEGY)) {
    WebSettingsCompat.setForceDarkStrategy(myWebView.getSettings(), WebSettingsCompat.DARK_STRATEGY_WEB_THEME_DARKENING_ONLY);
}

Note: Strategy selection requires Android System WebView v83 or up to be installed on the running device. WebView versions that support setForceDark but do not support strategy selections (v76 to v81) will use user agent darkening

The supported strategy options are:

• DARK_STRATEGY_USER_AGENT_DARKENING_ONLY: Only use user agent darkening and ignore any themes in the content (default)
• DARK_STRATEGY_WEB_THEME_DARKENING_ONLY: Only use the dark theme of the content itself to set the page to dark mode. If the content doesn't have a dark theme, the webview won't apply any darkening and show it "as is".
• DARK_STRATEGY_PREFER_WEB_THEME_OVER_USER_AGENT_DARKENING: Use the dark theme of the content itself to set the page to dark mode. If content doesn't have a dark theme, use user agent darkening.

How do Javascript checks work for darkened webviews?

The JavaScript call window.matchMedia('(prefers-color-scheme: dark)') will match in both the user agent darkening and web theme darkening strategy.

I have my webview set to FORCE_DARK_AUTO and my app is running in a daynight theme, but somehow my webview doesn't apply dark mode automatically based on my app theme. Why does this happen?

It's because the FORCE_DARK_AUTO setting value of the webview doesn't work based on themes (as noted in the documentation). It checks for the Android 10 Force Dark feature (a "quick-fix" dark mode feature for apps. It's similarly named, but not directly related to the WebView force dark).

If you aren't using force dark but a app theme to handle dark mode (as recommended), you have to implement your own check for when to apply the webview's force dark feature. An example when using a DayNight theme:

int nightModeFlags = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
if (nightModeFlags == Configuration.UI_MODE_NIGHT_YES) {
   //Code to enable force dark using FORCE_DARK_ON and select force dark strategy 
}