Well, the extension does an excellent job "darkenizing" most of the sites without losing readability.
But it does the reverse if applied to already dark-themed sites (either natively or via a Stylus theme). Also, sometimes text becomes unreadable and/or images invisible after applying dark mode. Manually whitelisting all such sites is cumbersome.
To deal with these issues, I would prefer any of the following mechanisms in this extension (or even better, all of them being configurable):
(a) have different mode for each tab;
(b) save last used mode for each site and apply it when opening it next time;
(c) use a list of sites to apply dark mode to (rather than not to apply it).