How can I trust that this extension, which has access to every page I visit, doesn't steal my data? How can I trust that if someone else takes over the project and releases an update, that my data is still secure?

Looks great, but I'm just so skeptical of browser extensions now.

https://github.com/darkreader/darkreader

However, I posed the same question in the MAS about the closed-source Safari version, which requires full access to webpage contents ("Can read sensitive information from webpages, including passwords, phone numbers, and credit cards on all webpages") and browsing history ("Can see when you visit all webpages").

The author's response boiled down to "trust me" and "trust Apple's review process".

Shortly after mentioning recent headlines highlighting weaknesses in Apple's review process ("More malicious apps found in Mac App Store that are stealing user data" https://appleinsider.com/articles/18/09/07/more-malicious-ap... , "Mac App Store apps are stealing user data" https://blog.malwarebytes.com/threat-analysis/2018/09/mac-ap... , etc), Apple deleted the review, but a cached version can be found here: http://www.gadgeteur.com/2018/11/26/dark-reader-for-safari-a... and here: https://pastebin.com/DxsWcaj7 .

EDIT: Other than the permissions issue (which was unfortunately a show-stopper for me), I was very pleased with the Safari extension's functionality; it could be a good fit for those who restrict their web browsing to non-sensitive sites or who can remember to disable it when necessary.