Google Chrome already supports dark mode on Windows and dark theme for websites, a properly dark mode supported website now appears complete dark in Chrome if Windows OS theme preference is set to dark, without relation to these, Chromium team now allows to force dark mode on all websites in the browser, the feature is now available behind a flag in the latest Canary and working.

“Force Dark Mode for Web contents” is the new flag added to Chrome flags page today, which when enabled “automatically render(s) all web contents using a dark theme”. The flag is available for Mac, Windows, Linux, Chrome OS, and Android.
Apart from the Enabled setting, Chrome allows to enable the feature with other following options:
Simple HSL-based inversion
Simple CIELAB-based inversion
Selective image inversion
Selective inversion of non-image elements
Selective inversion of everything
The options in the above will invert website light theme to dark, but the appearance of the page could be different when compared with Enabled setting.
To test the feature right away
1. Ensure you’re using latest Chrome Canary 78.0.3873.0 on desktop
2. Visit chrome://flags
3. Search for “dark mode” and for Force dark mode for Web contents flag, select “Enabled”.

4. Relaunch the browser, visit any website, the contents in it will go dark, to disable dark mode for websites you need to restore default value for the flag.
Related articles:
It is possible to Disable dark mode in Chrome only, but not for Windows 10
Chrome 75: First Run Experience gets a complete makeover, now supports dark mode too
Chrome Canary now supports dark mode in Windows 10 and respects light/dark mode Setting