Improve styling.

README.md

@@ -6,9 +6,9 @@ A proof-of-concept workspace built on
 **saved layouts** that open a whole arrangement — directories, scripts and all —
 in one go.
 
-The sidebar holds the window controls, a new-tab button, and one row per tab —
-the layout Zen Browser uses for vertical tabs — with the content inset to its
-right.
+The sidebar holds the window controls, a new-tab button, one row per tab, and a
+settings gear at its foot — the layout Zen Browser uses for vertical tabs —
+with the content inset to its right.
 
 ## Quick start
 
@@ -107,7 +107,8 @@ just the VT core.
 ## Architecture
 
 ```
-main.zig       AdwApplication, CSS loading
+main.zig       AdwApplication startup
+appearance.zig the colour scheme: preference -> libadwaita, CSS, palette
 Window.zig     sidebar + GtkStack of views, tab management, shortcuts
 View.zig       one tab's content: its panes, their layout, and drag handling
 Layout.zig     the split tree: nodes, rearranging, GtkPaned materialization
@@ -121,7 +122,9 @@ SaveLayoutDialog.zig  turns the current tab into a saved layout
 Session.zig    libghostty-vt Terminal + parser, fed by the PTY
 Pty.zig        openpt/fork/exec, controlling terminal setup
 key.zig        GDK keyval -> libghostty-vt key mapping
-theme.zig      colors libghostty-vt has no opinion about
+theme.zig      colors libghostty-vt has no opinion about, per scheme
+Settings.zig   preferences: model, JSON on disk
+SettingsDialog.zig    the settings page
 ```
 
 A tab is a **view**, and a view holds one or more **panes** arranged in a
@@ -286,6 +289,9 @@ in principle, but a terminal grid is small.
 - **Zooming a pane** to fill its tab and back, from the header button or
   `Ctrl+Shift+Z`. Nothing closes and nothing moves — hidden panes keep running
   and the split comes back exactly as it was. See [Zoom](#zoom)
+- **Light and dark schemes**, following the desktop by default and pinnable
+  from the settings page, applying to open tabs immediately — terminal palette
+  included. See [Theme](#theme)
 
 ### Shortcuts
 
@@ -299,6 +305,7 @@ in principle, but a terminal grid is small.
 | `Ctrl+Shift+V` | paste into a terminal (bracketed-paste aware, refuses unsafe pastes) |
 | `Ctrl+Shift+R` | rename the current tab (empty name = follow the terminal) |
 | `Ctrl+Shift+Z` | zoom the focused pane to fill the tab, and back |
+| `Ctrl+,` | settings |
 | `Ctrl+PageUp/PageDown` | previous / next tab |
 | `Alt+1`..`Alt+8` | jump to tab N, `Alt+9` jumps to the last |
 
@@ -468,6 +475,60 @@ smolvm machine exec --name NAME -- mkdir -p /root/.claude/hooks
 image that has no `jq`; if the title can't be read the state still reports and
 the tab simply keeps the name it had.
 
+## Theme
+
+Deep navy surfaces with Signal's ultramarine (`#3a76f0`) as the accent, in a
+light and a dark scheme. `Ctrl+,` or the gear at the foot of the sidebar opens
+**Settings**, which currently holds one choice: light, dark, or **system**,
+which is the default and follows the desktop.
+
+The preference lives in `~/.config/playpen/settings.json` (or
+`$XDG_CONFIG_HOME`), beside `layouts.json`:
+
+```json
+{
+  "version": 1,
+  "theme": "system"
+}
+```
+
+Switching applies immediately to every open tab — nothing needs restarting, and
+a shell that has been running all day repaints along with everything else.
+
+Three separate colour systems have to agree for that to be true, which is what
+`appearance.zig` exists to arrange:
+
+- **libadwaita's style manager** colours the stock widgets — popovers, entries,
+  dialog chrome. It is told to force a scheme, or left on `default` to follow
+  the desktop.
+- **`style.css`** colours everything the app draws itself. It is written
+  entirely against named colours, with one palette file per scheme
+  (`palette-dark.css`, `palette-light.css`); the matching palette is prepended
+  and the pair loaded as a single `GtkCssProvider`. No rule in `style.css` may
+  hardcode a colour — a literal hex is a rule that looks right in whichever
+  scheme you happened to be testing in.
+- **`theme.zig`** holds what Cairo draws the terminal grid from: the default
+  background, foreground and cursor, plus the 16 ANSI colours. The style tree
+  is never consulted there, so a CSS reload alone would leave every terminal
+  painted in the scheme it started in.
+
+The ANSI palette is the part that is easy to skip and shouldn't be. The
+standard xterm yellow is `#cdcd00`, which on a white background is close to
+invisible — and prompts and build tools use it constantly, so a light scheme
+without a light palette is a light scheme you can't read. The 240 colours above
+index 16 are fixed by spec and left alone; only the 16 named ones change.
+
+They are swapped through libghostty-vt's `DynamicPalette.changeDefault`, which
+changes what the palette *defaults* to. Anything a program set for itself with
+OSC 4 survives the switch, and a later OSC 104 reset returns to the current
+scheme's palette rather than the one the app happened to start in.
+
+Whether "system" currently means light or dark is libadwaita's answer, not
+ours: it already watches the desktop for the setting. So the stylesheet follows
+its `dark` property rather than the stored preference, and the preference only
+decides what the style manager is told. A desktop that switches at sunset takes
+this app with it, with no extra machinery and no second source of truth.
+
 ## Not implemented
 
 This is a proof of concept, and the following are deliberately absent: