Hi ReelButter,
thanks for your rating. It's great to hear that you like the add-on. 😀

The warning that the extension “is not actively monitored” is just a warning of Mozilla that explains the extension is not part of their suggested and extra-strongly vetted add-ons. See https://support.mozilla.org/kb/add-on-badges for more information.
However all add-ons are verified by Mozilla partly in an automated or manual way, see https://extensionworkshop.com/documentation/publish/signing-and-distribution-overview/#post-submission-review for details about that.

Anyway, as for this extension, you can be sure it is not malicious, because you can also check it's source code at https://github.com/rugk/website-dark-mode-switcher.

I hope this explanation helped and continue to have fun using the extension.

Best regards,
rugk