Catppuccin Mocha theme and a legible cursor

Mocha with the palette's red as the accent is now the default; the old
green scheme moves to `:set theme=green` alongside amber and ice.

Two readability fixes in the editor:

- The vim block cursor paints the character inside itself in that
  character's own syntax colour, so an accent-coloured glyph — a
  heading's `#`, a link — vanished against the accent-coloured block.
  The glyph is now forced to the background colour.
- CodeMirror injects its own `.cm-activeLine` rule after this stylesheet,
  so the equally specific override never applied and the current line
  rendered as a bright blue box. Qualified the selector and toned the
  band down to a faint tint of the foreground.

Insert mode also gets a 2px bar instead of the browser hairline caret.

README.md

@@ -59,7 +59,7 @@ The desktop UI is modal. In the browse pane:
 Inside the editor the full vim keymap is live — `w`/`b`, `dd`, `ciw`, visual
 mode, macros, `:w` to write. `:help` lists every ex command; the useful ones are
 `:w`, `:wq`, `:q!`, `:new [yyyy-mm-dd]`, `:date`, `:title`, `:share`, `:link`,
-`:upload`, `:media`, `:search`, `:set theme=green|amber|ice` and `:set novim`.
+`:upload`, `:media`, `:search`, `:set theme=mocha|green|amber|ice` and `:set novim`.
 
 On a touch device modal editing is turned off and the same actions are buttons.