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]>

DESIGN.md

@@ -437,12 +437,31 @@ document has a portal pointing here.
 ```
 
 **Config** (TOML) lives in `%APPDATA%\cumulus\config` (`~/.config/cumulus` on
-Linux); `cumulus --paths` prints the folders. `:config` writes these files with
-a comment on every option, and cumulus reloads them when they change.
+Linux); `cumulus --paths` prints the folders. Each file only states what it
+changes: a missing file changes nothing, and a mistake only drops its own line,
+with a message in the status bar saying what's wrong. cumulus applies a file
+again when it's saved. (Settled in part 8b.)
+
+- `:config` lists config.toml, keys.toml and the theme's file, with the folder
+  they're in; Enter opens one in your editor (the `editor` setting, `$VISUAL`,
+  `$EDITOR`, else notepad on Windows and vi elsewhere), with the terminal
+  handed over until it's done. A file that isn't there yet is written first,
+  every option at its default behind a `#`, with what it does: keys.toml lists
+  every command by name.
+- `:set` and `:theme` change things until cumulus quits, as in vim; the files
+  change them for good.
+- A theme file is laid over the built-in theme (`default`): a theme that says
+  just `accent = "magenta"` is the built-in one in magenta, and
+  `themes/default.toml` changes the built-in theme itself. Screen colors are
+  terminal colors; the colors in looks and styles are palette names.
+- The built-in theme comes with named styles, each put on by the tag of the
+  same name: `question` (yellow, dashed), `important` (red, thick) and `done`
+  (gray, its title faded); and two for links, `blocks` (red, dashed, "blocks")
+  and `supports` (green, "supports"). The user's pick in part 8b.
 
 ```toml
 # config.toml
-theme = "default"            # a file in the themes folder
+theme = "default"            # or a file in the themes folder
 default_document = "ideas"   # what `cumulus` looks for at startup
 autosave = true
 mouse = true
@@ -451,11 +470,11 @@ which_key_delay = "300ms"
 max_node_width = 30
 glyphs = "unicode"           # "ascii" for fonts without box-drawing characters
 completion_popup = "as-you-type"  # "on-tab": only after Tab, like vim's wildmenu
+editor = ""                  # what :config opens files in ("nvim", "code --wait")
 
 # keys.toml: sections motions, normal, canvas, outline, note, visual, move,
 # connect, list, card, text (vim keys inside text), insert (typing: one key each,
-# the others type themselves); commands by name (src/input/command.rs);
-# "nothing" unbinds
+# the others type themselves); commands by name; "nothing" unbinds
 leader = "<Space>"
 [normal]
 "<Space>x" = "delete-branch"
@@ -463,23 +482,37 @@ leader = "<Space>"
 [canvas]
 "H" = "previous-document"
 
-# themes/default.toml
-accent = "cyan"
-[palette]                    # terminal names, 0-255, or "#rrggbb"
-red = 167
-orange = 208
-green = 108                  # … yellow, teal, blue, purple, pink, brown, gray
+# themes/night.toml
+accent = "cyan"              # screen colors: terminal names, 0-255, or "#rrggbb"
+muted = "darkgray"
 [modes]
 insert = "green"
 visual = "magenta"
-[styles.node.question]
-color = "yellow"
+[palette]                    # the colors nodes and lines are given, by name
+red = 167
+orange = 208                 # … yellow, green, teal, blue, purple, pink, brown, gray
+[node]                       # every node, unless something says otherwise
+border = "square"
+[root]
+border = "rounded"
+[branch]                     # lines from a parent to its children
+line = "solid"
+arrows = "off"
+color = "node"               # the parent's own color
+[link]                       # cross-links
+line = "dashed"
+arrows = "end"
+[styles.node.question]       # named node styles: <Space>s, and [tags]
+color = "yellow"             # a palette color
 border = "dashed"
-[styles.link.blocks]
+[styles.node.done]
+color = "gray"
+faded = true                 # the title in the muted color
+[styles.link.blocks]         # named line styles, for links
 color = "red"
 line = "dashed"
 arrows = "end"
-label = "blocks"
+label = "blocks"             # shown on a link without a label of its own
 [tags]
 question = "question"        # a #question node gets the "question" style
 ```
@@ -523,22 +556,24 @@ cumulus/
    │               motion, object, layout), list (the popup list help, menus and pickers
    │               share), card (the K card), status_bar, document_bar, which_key,
    │               command_line, theme, glyphs
-   ├─ config/      paths (the config and data folders), settings (and `:set`); later keys_file,
-   │               theme_file (TOML)
+   ├─ config/      paths (the config and data folders), settings (config.toml, `:set`), keys_file
+   │               (keys.toml's template), theme_file (themes laid over the built-in one)
    ├─ storage/     document_file (RON), write_atomically, names (file names from titles), backup,
    │               lock, stamp (noticing changes made outside), memory (memory.ron)
    └─ app/         mod.rs (App), input (keys and the mouse, routed), update (carrying actions out),
                    render, terminal (the main loop, setup, teardown, panic hook), startup (what
                    `cumulus` opens), documents (the open documents and their files), files
-                   (saving, reloading, locks, where you left off), action, mode, buffer, commands
+                   (saving, reloading, locks, where you left off), configure (settings, keys and
+                   themes applied; `:config`), action, mode, buffer, commands
                    (keys every view shares), calls (`:` commands), text_edit (a node's text being
                    edited, rapid entry), registers, panes (the split tree), popup (help, menu,
                    pickers, card, questions); later jumps, portals (cache, backlinks, renames)
 ```
 
-The end-to-end tests are in `app/tests.rs` (and `app/tests/files.rs`): they
-copy the sample documents to a temporary folder, type keys and click on a test
-terminal, and check the screen, the documents and the files.
+The end-to-end tests are in `app/tests.rs` (with `app/tests/files.rs` and
+`app/tests/config.rs`): they copy the sample documents to a temporary folder,
+type keys and click on a test terminal, and check the screen, the documents
+and the files.
 
 **Flow:** a terminal event comes in → the key engine turns keys into a typed
 `Command` (with its count and register) → the focused view answers it with