Warning: Do not switch away from Smooth Sailing!
The Smooth Sailing theme and the Flexible Squares theme have been discontinued by LiveJournal.
If you are currently using a layout based on Smooth Sailing you can continue to do so. But you can not choose it if you do not currently have it. If you switch away from Smooth Sailing to another theme, even just to try, you can not get Smooth Sailing back. If you are contemplating a new layout choose one that is also based on Smooth Sailing and only change your custom CSS.
sources/further reading:
https://mintyapple.livejournal.com/842475.html
https://revolumna.livejournal.com/6279.html
https://layout-lounge.livejournal.com/61545.html
Update: If you have a paid account you can recreate your previous theme:
https://layout-lounge.livejournal.com/61545.html
If you are currently using a layout based on Smooth Sailing you can continue to do so. But you can not choose it if you do not currently have it. If you switch away from Smooth Sailing to another theme, even just to try, you can not get Smooth Sailing back. If you are contemplating a new layout choose one that is also based on Smooth Sailing and only change your custom CSS.
sources/further reading:
https://mintyapple.livejournal.com/842475.html
https://revolumna.livejournal.com/6279.html
https://layout-lounge.livejournal.com/61545.html
Update: If you have a paid account you can recreate your previous theme:
https://layout-lounge.livejournal.com/61545.html