You seem to be referring to the ios/mac app store thing specifically, which is NOT what 99% of people use. That is a separate whole browser with the extension pre-installed and completely unnecessary.

Just use the links in the middle of the page and it will install it for free like every other normal browser extension people use all the time.

https://addons.mozilla.org/en-US/firefox/addon/darkreader/

The firefox extension alone has 1 million installs just from this page, we all installed it for free and have been using it for years. I think asserting that there are only paid options is disingenuous to say the least.