Read config.toml, keys.toml and themes, and open them with :config (stage 8b)

The config folder now sets everything: config.toml the settings (now
also `theme` and `editor`), keys.toml the keys, and themes/<name>.toml
the theme, laid over the built-in one, so a theme only needs what it
changes. Each file is applied again a second after it's saved; a
mistake only drops its own line, and the status bar says what's wrong,
naming the file.

:config lists the three files with their folder; Enter hands the
terminal to your editor (the editor setting, $VISUAL, $EDITOR, else
notepad or vi), writing the file first if it's missing, with every
option at its default behind a #, explained (keys.toml lists every
command by name). :theme switches themes until cumulus quits, like
:set.

The built-in theme gets the named styles the user picked, each also put
on by its tag: question (yellow, dashed), important (red, thick) and done
(gray, its title faded); and link styles blocks and supports, whose
label shows on a link without its own. Themes can also set [modes],
[node], [root], [branch] and [link].

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -42,6 +42,23 @@ saves a second after each change, reloads a file another program changed
 A document open in another cumulus opens read-only. Each one reopens where you
 left it.
 
+## Settings, keys and themes
+
+`:config` lists the three config files: `config.toml` (the settings),
+`keys.toml` (your keys) and the theme's file. Enter opens one in your editor
+(the `editor` setting, or `$VISUAL` / `$EDITOR`, or notepad / vi); the first
+time, it's written with every option at its default behind a `#`, explained.
+Take a `#` away, save, and cumulus applies it at once. Each file only needs
+what it changes, and a mistake only skips its own line (the status bar says
+what's wrong).
+
+- `:set name=value` changes a setting until cumulus quits (`:set name?` shows
+  it, Tab lists them).
+- `:theme name` switches to `themes/name.toml`, laid over the built-in theme:
+  a theme can say just `accent = "magenta"`.
+- The built-in theme has named styles (`<Space>s`), also put on by tags:
+  `#question` (yellow, dashed), `#important` (red, thick), `#done` (faded).
+
 ## Keys
 
 The plan for the everyday keys (all of them are in DESIGN.md):