Before removing Stylish, generate a “Backup” file you can use to move your user styles (themes) to another extension. A popular one is Stylus (on the Add-ons site) but there are a few others as well (list on Github; see also Can’t Install Stylish (July 2018) on userstyles.org forums).
Images missing below? See: https://www.userchrome.org/stylish-to-stylus.html
Here’s the process:
(1) Call up Stylish’s “Manage All Styles” page
Note: You might need to re-enable Stylish on the Add-ons page first.
(2) Click the “Backup all Installed” button and then Save the file
(3) Open the Downloads folder to the backup file using the “Open Containing Folder” button on the Downloads panel
(4) If you haven’t already, install the Stylus extension
(5) Call up Stylus’s Manage page
(6) Position the window with the Downloads folder so you can drag and drop the Stylish backup file onto the Stylus Manage page to import your styles
That’s it. You can disable (or remove) Stylish now.