From How to force enable dark mode for web contents in Microsoft Edge? (https://www.kunal-chowdhury.com/2020/04/how-to-force-enable-dark-mode-for-web-pages.html):
QuoteForced dark mode for web contents is a feature that came with Google Chrome version 78. As the new Microsoft Edge browser also uses the chromium engine, it also inherits this feature. Many of you might not know about this feature and how to enable it in Microsoft Edge.
This feature can be turned ON by enabling a browser flag.
To find out how to enable this setting in the referenced article scroll down to "How to enable dark mode for web contents in Microsoft Edge?" After following those steps, Microsoft Edge will automatically apply dark mode to all web pages.
Another option I just found is an extension for Microsoft Edge: Dark Mode (https://microsoftedge.microsoft.com/addons/detail/dark-mode/boldmdfoencgjfblcelefkjfafmpiahm). Clicking the icon in the taskbar is an on/off switch so you can switch back to light mode for a site that doesn't display well in dark mode.