How do I update the built-in Style Sheets?
MultiMarkdown Composer comes with several Style Sheets built-in. These will sometimes be updated with new releases of the app.
Whenever you select one of the built-in Style Sheets for the first time, a copy is put into your "Styles" folder (easily found via the Appearance panel in the Preferences window). This allows you to edit a copy to customize as you like. If you have a local copy, Composer will use that instead of the copy embedded inside the app. Composer will not overwrite your copy so that it won't lose any changes you have made.
If you want to refresh your local built-in style sheets, you can simply remove them from the Styles Folder, and the next time you choose one, it will be refreshed from the app.