docs: remove outdated theme section as system theme is now added back (#5041)

packages/web/src/content/docs/1-0.mdx

@@ -44,10 +44,6 @@ We removed some functionality that we weren't sure anyone actually used. If some
 
 ## Breaking changes
 
-### Theme
-
-The `system` theme has not yet been ported and custom themes aren't loaded yet but both of these will be fixed this week.
-
 ### Keybinds renamed
 
 - messages_revert -> messages_undo