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Here are some unofficial solutions.
You can install a different extension with similar features, and then install the theme.
Forum users most often suggest Stylus, which is available from:
Another option is xStyle, which is available from:
You do not need to reinstall. You can re-enable Stylish on your browser's Extensions page:
Note: You may wish to review your settings related to data sharing. These are set on the "Manage All Styles" page in Stylish. It's the checkbox for "Send anonymous data to Stylish developers" — that might not be as anonymous as it sounds.
There is no official repository for old versions of the Stylish extension. Users have shared the following unofficial links, which you can use at your own risk.
That is the subject of many other threads you can find further down the forum. You can review Mozilla's notice here: https://blocked.cdn.mozilla.net/c635229f-7aa0-44c5-914f-80c590949071.html.