theme: a name and a directory, not sixty colours in your config

Wearing a theme meant pasting its `theme { }` block into config.kdl, or
including a path to a file you had copied there yourself. Both make the
palette a literal in your config, and a literal is the one thing you cannot
change cheaply: not from a script, not from a running session, not by a list
of what is available, and never again from upstream -- the colours you pasted
are the colours that theme had the day you pasted them.

So a theme is a file in a directory now, and a config names it:

    theme_name "phosphor"
    theme { frame_focus "#00ff88" }   // ...but that one colour is mine

theme_name is applied after the includes and before everything the file says
itself, which is the rule includes already follow and for the same reason: the
loader reads a document by asking it for names rather than walking it in order,
so "here" is not a position it could honour, and "what you asked for is the
base, what you wrote is yours" is the rule worth having.

Two directories, yours first: `themes/` beside the root config -- beside the
file that was loaded, so a config somewhere else (SLOSH_CONFIG, or one handed
to --check) takes its themes with it -- and then <prefix>/share/slosh/themes,
compiled in from PREFIX. install(1) has been putting themes there all along
and nothing could read them; now a shipped theme is nameable without being
copied, and a file of yours with the same name shadows it, which is how a
root-owned theme gets edited.

A name, not a path. The directory is what makes a theme switchable, listable
and shadowable, and a path silently does none of those, so `theme_name
"../x.kdl"` is refused with a line pointing at `include`, which still works
and is still the answer for a file somewhere a directory of names cannot
reach. A theme that is not there is one complaint and no more (D9): the rest
of the config applies, because losing your keybindings over a mistyped colour
scheme would be the wrong trade. A theme file is read by the same loader, so
it is watched like any other: saving one reloads the session wearing it, and
the derivations (tab_status_fg, tab_idle_bg) run over the theme's own colours
exactly as before.

Switching without editing anything, over the socket a session already has:

    slosh -s main cmd theme                                # amber default* ...
    slosh -s main cmd '{"cmd":"theme","name":"amber"}'      # now
    slosh -s main cmd '{"cmd":"theme","name":"amber","save":true}'

The session holds the name over later reloads, so saving your config for an
unrelated reason does not put the old colours back; `save` writes the
theme_name line into the file, replacing the one already there and leaving
every other line and comment alone -- dumping the whole config back over
itself would reformat a hand-written file and drop its comments, which is not
a fair price for changing a colour scheme. A name is tried on a throwaway
config first, so a typo costs nothing. An empty name gives the answer back to
the file.

--dump-config stops writing colours: it writes the theme's name and the
colours a config overrode by hand, so a dumped config keeps following its
theme instead of freezing it. --dump-theme is the other half and is new: the
resolved palette as a theme file, which is how you start one from the session
in front of you rather than from an empty file. It is also what keeps
contrib/themes/default.kdl honest -- that file is now the annotated reference
(the commented palette moved there out of config.kdl, where it was 119 lines
of colour in the middle of the prose), and a test compares it against the
compiled-in one.

While in there: tests/test_themes.py's "every theme sets every colour" guard
had been grepping for `&c->`, the spelling before the colour table moved to
offsetof, so it had been finding zero colours and passing for months. Fixed,
with the derived and opt-in colours exempted for the reasons they are derived
and opt-in, and with a check that the guard found a table at all.

1 files changed, 17 insertions, 0 deletions

diff --git a/include/app.h b/include/app.h
index 106d54a..74b841d 100644
--- a/include/app.h
+++ b/include/app.h
@@ -59,6 +59,23 @@ char *app_take_clipboard(app_t *a);
 
 /* Re-read the config file. Keeps the working one if the new file is broken. */
 bool app_reload_config(char *err, size_t errcap);
+
+/* Switch the session to a named theme, now.
+ *
+ * The name is held by the session and re-applied over every later reload, so
+ * editing your config does not quietly put the old colours back. `save` also
+ * writes the `theme_name` line into the config file, which is the difference
+ * between "for now" and "from now on" -- a live switch that edited your config
+ * behind you would be rude, and one that vanished on the next reload would be
+ * useless, so it is a choice the caller makes. An empty name drops the
+ * session's own answer and goes back to whatever the file says. */
+bool app_set_theme(const char *name, bool save, char *err, size_t errcap);
+/* The theme in force, or "" when nobody has named one. */
+const char *app_theme(void);
+/* What there is to pick from, and where it was looked for. Through the app
+ * rather than the config, because the front ends do not own a config. */
+size_t app_themes(char (*out)[64], size_t max);
+size_t app_theme_dirs(char (*out)[512], size_t max);
 /* The files the config in force was read from: the one that was loaded and
  * everything it included, existing or not. What the watcher watches. */
 size_t app_config_files(const char **out, size_t max);