From the OP on /r/redditsync. Thanks to /u/ljdawson for all of the great work put into this release!

v12.1

Major

• Added support for per-account settings
• Added multireddit management (add or remove subs from a multi)
• Added a brand new Theme customization menu to the settings complete with color picker
• Added lots more mod tools!
• Added threaded messaging
• Super speedy table support in comments (no lagging for huge tables)
• Access your "recently viewed" to keep track of multiple threads
• Added support for reddit "seen it" to spot duplicate submissions (comments > seen it)
• Added support for more image providers to the in-app image viewer
• Continued speed and performnce improvements
• Sync is now 99.8% crash free!

New

• Replaced the Toolbar options with Theme customization
• Added a confirm popup when marking all messages as read
• Added support for tables in comments (/u/Mynameisnotdoug)
• Added a color picker to the settings
• Added a left handed mode to comments to reverse comment actions
• Added support for old imgur galleries (hurrah)
• Multireddit contents now show in the sidebar
• Subreddits can now be added to a multireddit
• Subreddits can now be removed from a multireddit
• Added "recently viewed" to the new comments drawer (swipe right) and the posts overflow
• CSS user flairs are now displayed in comments (optional setting and disabled by default)
• MOD tools: added the ability to ban users (requires logging in again for new permissions)
• MOD tools: comments in subs moderated by user can now be accessed
• MOD tools: comments can now be stickied and distinguished
• Distinguished and stickied comments now show markers

Changes

• Added support for XKCD images in the image viewer
• Images should now show up faster when scrolling through posts
• Added icons to the top of the friend and trending drawers
• Added the ability to quickly see the sidebar of a trending subreddit from the trending sidebar
• Added the ability to see the sidebar of a sub from the overflow of a comments page
• Trending subs can now be copied by long pressing the subreddit
• Recently viewed subs can now be copied by long pressing the title
• Adjusted the upvote and downvote buttons for the comments left handed mode
• Added support for Giphy GIFs that do not have an MP4 version
• Added an option to only check messages as the app starts
• The type button can now be used to peek (/u/Muunuu)
• Changelogs are no longer shown if you restore from a previous backup
• Self hosted reddit GIFs now serve MP4 variants!

Fixes

• Enter / exit animations are now tidied up after they have completed. This should fix the N split screen issues.
• Removed the enter / exit animation when rotating or resuming the posts and comments views
• MOD tools: fixed an issue with pagination not loading correctly for all mod sections
• Fixed an issue with full width padding not applying to small cards

Closed

• https://github.com/laurencedawson/reddit-sync-development/issues/359
• https://github.com/laurencedawson/reddit-sync-development/issues/437
• https://github.com/laurencedawson/reddit-sync-development/issues/705
• https://github.com/laurencedawson/reddit-sync-development/issues/719
• https://github.com/laurencedawson/reddit-sync-development/issues/794
• https://github.com/laurencedawson/reddit-sync-development/issues/874
• https://github.com/laurencedawson/reddit-sync-development/issues/1080
• https://github.com/laurencedawson/reddit-sync-development/issues/1115
• https://github.com/laurencedawson/reddit-sync-development/issues/1126
• https://github.com/laurencedawson/reddit-sync-development/issues/1143
• https://github.com/laurencedawson/reddit-sync-development/issues/1213
• https://github.com/laurencedawson/reddit-sync-development/issues/1298
• https://github.com/laurencedawson/reddit-sync-development/issues/1417
• https://github.com/laurencedawson/reddit-sync-development/issues/1448