Toggle state in the icon's shape, not the theme's colours (#74)

4ed2d59 drew a pressed toggle in each theme's accent colour; the themes'
colours were not to change. Back as they were, pinned rows included. The
read button carries its state in its shape instead, as the pin does with
outline and solid: a tick when read, the envelope when not, where before
it showed the action (the envelope on a read item).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -30,9 +30,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-- Pinned and read look the same wherever they are shown, in every theme: the button lights up
-  in the theme's accent colour when it is on. The read button used to show an envelope on a
-  read item, the opposite of how the pin beside it worked.
+- The read button shows whether an item is read, as the pin beside it shows whether it is
+  pinned: a tick when read, an envelope when not. It used to show the opposite.
 - In Directory and Popular, Subscribe is a plus again, not the tick that marks a feed you have.
 - A web page with some non-ASCII characters no longer hides the feed it links, or crashes looking for it.
 - Pulling the item list down to check for new items shows a spinner for a couple of seconds, and