UI: seven themes, each dark and light, chosen in the Settings

Settings > Theme and Settings > Light or dark pick the colours of every
screen (issue #10): roro9stack (the website's, the default), Catppuccin,
Dracula, Nord, ANSI terminal, Gruvbox and Solarized.

- A palette is 14 roles (lib/ui/src/palette.h). theme::kText and the
  others are now references into the one in use, so the Apps are
  unchanged; the main loop applies it when a Setting changes.
- Every colour is one the RGB332 frame buffer holds exactly: the themes'
  own were rounded, and those that fell together or lost contrast were
  picked by hand. test_palette checks exactness and 18 contrast ratios
  for each palette.
- The Sky view's constellation colours come from the theme.
- `theme [0-6] [light|dark]` on the consoles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

site/content/guide/settings.md

@@ -16,6 +16,8 @@ Move with the arrows. On a toggle, a choice or a slider, left and right change t
 | **Brightness** | A slider |
 | **Dim after**, **Screen off after** | Screen timeouts. Dimming must come before the screen turns off |
 | **Launcher** | The home screen as a **Grid** of icons or a **List** of names (see [The basics](/guide/basics/#the-launcher)) |
+| **Theme** | The colours of every screen: **roro9stack** (the website's), **Catppuccin**, **Dracula**, **Nord**, **ANSI terminal**, **Gruvbox** or **Solarized** |
+| **Light or dark** | Which side of the theme: every one has both |
 | **Sound & LED** | The beep and the flash on a [Toast](/guide/basics/) |
 | **GNSS** | Switches the receiver on and off (see [GNSS](/guide/gnss/)) |
 | **Pause GNSS for LoRa** | Puts the receiver on standby while the radio listens (see [LoRa Scanner](/guide/lora-scanner/)) |
@@ -49,3 +51,11 @@ Two DNS servers (9.9.9.9 and 1.1.1.1 by default), used on Fixed networks, or on
 What <kbd>Fn</kbd> + <kbd>h</kbd> shows on these screens. These tables are generated from the firmware's own lists, so they are always the current ones.
 
 {{ keys(scopes=["settings", "settings-choice", "wifi", "wifi-servers", "wifi-network", "wifi-status", "wifi-scan", "wifi-name", "vpn", "debug-console"]) }}
+
+## Themes
+
+A theme changes the colours and nothing else: the fonts and the layout stay. It applies at once, to every App, and is kept across restarts.
+
+The screen shows **256 colours**, so a theme here is the nearest the screen can do to the palette it is named after, not an exact copy: pale backgrounds turn white or cream, and dark ones turn a deep blue or olive. Each theme is checked for contrast after that rounding.
+
+Three things keep their own colours whatever the theme: the SSH terminal (the 16 colours a remote program asks for), the Sweep's waterfall (its colours are a scale of signal strength), and pictures.