Open up about:config and go to layout.css.prefers-color-scheme.content-override and set it to one of the following:
0 => Force a Dark background
1 => Force a Light background
2 => Follow the System theme (default in Firefox 94)
3 => Follow the Browser toolbar theme (default in Firefox 95+)
You can also use Dark Reader in a reverse way by forcing everything into a light theme, or sepia if that makes reading better for you.
I don’t want pure light mode. I just want the defaults for each website.