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At this time, there are no links to source code anywhere easily visible on the UserStyles website for either the UserStyles server, or the Stylish extension. Is UserStyles/Stylish still open source at all?
There are posts on this forum which reference the Github repository stylish-userstyles/userstyles as though it were the official repository; although this repository does not contain any activity after 2016-12-09, and the latest commit from Jason Barnabe (the original developer) was on 2015-06-26.
The companion repository for the Stylish client on this Github account, stylish-userstyles/stylish, lists the latest commits as from Jason Barnabe on 2016-08-01, around the time the project ownership was transferred to Justin Hindman. Most confusing about all of this is that the latest release listed in this repository is 2.0.3, even though its latest commit is just days before the release of 2.0.7 on the Firefox addons site.
Also on this account, there is a repository for the chrome extension, stylish-userstyles/stylish-chrome, whose latest commit is from Justin Hindman (the current lead developer last I heard) on 2017-03-16. However, this commit is about version 1.7.6, the latest release in this repository is 1.5.1, yet the current release on the chrome store today is 1.8.3?
I can't make sense of this.