In Joplin, go to Tools → Options → Appearance → Show Advanced Settings and click "Custom stylesheet for rendered Markdown". The file should open in your default text editor. Copy and paste the whole code from https://discourse.joplinapp.org/t/typographical-line-spacing/8071/6 (already linked above) into it, then save changes and exit. Next, restart Joplin (make sure to exit it completely first, including the tray icon).