For now there are plug-in for browsers available (seen in some other threads here)
But it might be coming at some time, they still are building this site at the moment
Here is a workaround.
Using the Chrome browser, install the "Dark Mode" extension.
Click on the "manage extensions" usually on the top right of Chrome.
You should see "Dark Mode".
Toggle to the mode you want at will.
In the setting options of Dark Mode (click on the three vertical dots), you can set an "include" and/or "exclude" list of websites you want to be in dark mode.
I using Firefox you can use the Extension/AddIn "Dark Reader". Works fine. Now, all my internet is dark. So much better to read and maybe even more healthy in terms of reduced blue radiation, which could harm my day-night rhythm and related hormone system (especially when being online late in the evening).