feat(tui): add "Terminal" theme that fully inherits the host terminal's colors

Adds a new selectable theme `terminal` (alongside System / Whale / Whale
Light / Grayscale / Catppuccin / Tokyo Night / Dracula / Gruvbox) that
paints every surface with `Color::Reset` instead of any RGB so the
host terminal's own background, foreground, and palette show through.

The existing `system` theme only chose between two RGB themes (Whale dark
or Whale Light) based on COLORFGBG / macOS appearance — useful, but it
still painted brand-colored RGB surfaces. Users with custom terminal
themes (Solarized, Nord, transparent backgrounds, custom Ghostty/iTerm
schemes) had no way to make the TUI respect their terminal palette.

Implementation:
- New `TERMINAL_UI_THEME` const where every `*_bg` and most text slots
  are `Color::Reset`, and accents (mode_agent/yolo/plan, status_working,
  status_warning) use ANSI named colors so they also inherit the user's
  terminal palette rather than DeepSeek brand RGB.
- `ThemeId::Terminal` plumbed through `from_name` / `name` /
  `display_name` / `tagline` / `ui_theme` / `SELECTABLE_THEMES`, and
  registered in `normalize_theme_name` with aliases `term`,
  `transparent`, `follow-terminal`, `inherit` so existing
  user-friendly config strings just work.
- `theme_remap_active(Terminal) → true` so the existing per-cell remap
  in `ColorCompatBackend` rewrites every hard-coded palette constant
  (`DEEPSEEK_INK`, `DEEPSEEK_SLATE`, `BORDER_COLOR`, `TEXT_BODY`, …) to
  `Color::Reset`. Without this, the many render sites that reach for
  the named palette constants directly would still paint brand RGB.
- `theme_green` / `theme_red` return `Color::Green` / `Color::Red`
  for Terminal so diff "+"/"−" stay green/red but follow the user's
  terminal palette.
- `theme_diff_added_bg` / `theme_diff_deleted_bg` return `Color::Reset`
  for Terminal — diff highlight is conveyed by foreground color only.
- The new theme is the second entry in `SELECTABLE_THEMES` (right after
  System) so it surfaces prominently in the `/theme` picker.

theme_picker tests: the new theme is inserted in row 2 of
`SELECTABLE_THEMES`, which shifts the indices three existing tests
relied on — `arrow_down_previews_next_theme`,
`enter_commits_with_persist_true`, and `digit_jumps_to_row` — so those
expectations are updated to match the new ordering. No production
behavior change in those tests, just index arithmetic.

Default (`theme = "system"`) is unchanged; existing users see no
difference. Users who want full terminal pass-through opt in via
`/theme` or `theme = "terminal"` in settings.toml.

crates/tui/src/palette.rs

@@ -825,6 +825,39 @@ pub const DRACULA_UI_THEME: UiTheme = UiTheme {
     tool_failed: Color::Rgb(0xff, 0x55, 0x55),  // red
 };
 
+/// "Terminal" theme: lets the host terminal's color scheme show through
+/// instead of painting any RGB surface. Backgrounds use `Color::Reset`
+/// (the terminal's own default bg) and most text uses `Color::Reset`
+/// (terminal's own default fg). Accents are ANSI named colors so they
+/// also inherit the user's terminal palette (Solarized, Nord, custom
+/// schemes, etc.) rather than DeepSeek brand RGB.
+pub const TERMINAL_UI_THEME: UiTheme = UiTheme {
+    name: "terminal",
+    // Mode is reported as Dark to avoid the dark→light cell remap kicking
+    // in; the terminal-theme cell remap already normalizes everything to
+    // `Color::Reset`, and we never want a second pass overwriting that.
+    mode: PaletteMode::Dark,
+    surface_bg: Color::Reset,
+    panel_bg: Color::Reset,
+    elevated_bg: Color::Reset,
+    composer_bg: Color::Reset,
+    selection_bg: Color::Reset,
+    header_bg: Color::Reset,
+    footer_bg: Color::Reset,
+    mode_agent: Color::Blue,
+    mode_yolo: Color::Red,
+    mode_plan: Color::Yellow,
+    status_ready: Color::Reset,
+    status_working: Color::Cyan,
+    status_warning: Color::Yellow,
+    text_dim: Color::Reset,
+    text_hint: Color::Reset,
+    text_muted: Color::Reset,
+    text_body: Color::Reset,
+    text_soft: Color::Reset,
+    border: Color::Reset,
+};
+
 pub const GRUVBOX_DARK_UI_THEME: UiTheme = UiTheme {
     name: "gruvbox-dark",
     mode: PaletteMode::Dark,
@@ -874,6 +907,7 @@ pub const GRUVBOX_DARK_UI_THEME: UiTheme = UiTheme {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ThemeId {
     System,
+    Terminal,
     Whale,
     WhaleLight,
     Grayscale,
@@ -891,6 +925,7 @@ impl ThemeId {
     pub fn from_name(value: &str) -> Option<Self> {
         match normalize_theme_name(value)? {
             "system" => Some(Self::System),
+            "terminal" => Some(Self::Terminal),
             "dark" => Some(Self::Whale),
             "light" => Some(Self::WhaleLight),
             "grayscale" => Some(Self::Grayscale),
@@ -908,6 +943,7 @@ impl ThemeId {
     pub const fn name(self) -> &'static str {
         match self {
             Self::System => "system",
+            Self::Terminal => "terminal",
             Self::Whale => "dark",
             Self::WhaleLight => "light",
             Self::Grayscale => "grayscale",
@@ -923,6 +959,7 @@ impl ThemeId {
     pub const fn display_name(self) -> &'static str {
         match self {
             Self::System => "System",
+            Self::Terminal => "Terminal",
             Self::Whale => "Whale (Dark)",
             Self::WhaleLight => "Whale Light",
             Self::Grayscale => "Grayscale",
@@ -938,6 +975,9 @@ impl ThemeId {
     pub const fn tagline(self) -> &'static str {
         match self {
             Self::System => "Follow terminal background (COLORFGBG / macOS appearance)",
+            Self::Terminal => {
+                "Inherit terminal colors fully (transparent surfaces, ANSI accents)"
+            }
             Self::Whale => "Whale dark — deep navy & gold",
             Self::WhaleLight => "DeepSeek light, paper-ish",
             Self::Grayscale => "Color-minimal high contrast",
@@ -956,6 +996,7 @@ impl ThemeId {
     pub fn ui_theme(self) -> UiTheme {
         match self {
             Self::System => UiTheme::detect(),
+            Self::Terminal => TERMINAL_UI_THEME,
             Self::Whale => UI_THEME,
             Self::WhaleLight => LIGHT_UI_THEME,
             Self::Grayscale => GRAYSCALE_UI_THEME,
@@ -970,6 +1011,7 @@ impl ThemeId {
 /// Themes shown in the `/theme` picker, in display order.
 pub const SELECTABLE_THEMES: &[ThemeId] = &[
     ThemeId::System,
+    ThemeId::Terminal,
     ThemeId::Whale,
     ThemeId::WhaleLight,
     ThemeId::Grayscale,
@@ -1012,6 +1054,7 @@ impl UiTheme {
 pub fn normalize_theme_name(value: &str) -> Option<&'static str> {
     match value.trim().to_ascii_lowercase().as_str() {
         "" | "auto" | "system" | "default" => Some("system"),
+        "terminal" | "term" | "transparent" | "follow-terminal" | "inherit" => Some("terminal"),
         "dark" | "whale" | "whale-dark" => Some("dark"),
         "light" | "whale-light" => Some("light"),
         "grayscale" | "greyscale" | "gray" | "grey" | "mono" | "monochrome" | "black-white"
@@ -1189,7 +1232,11 @@ const fn theme_diff_deleted_bg(ui: &UiTheme) -> Color {
 pub const fn theme_remap_active(theme: ThemeId) -> bool {
     matches!(
         theme,
-        ThemeId::CatppuccinMocha | ThemeId::TokyoNight | ThemeId::Dracula | ThemeId::GruvboxDark
+        ThemeId::Terminal
+            | ThemeId::CatppuccinMocha
+            | ThemeId::TokyoNight
+            | ThemeId::Dracula
+            | ThemeId::GruvboxDark
     )
 }
 

crates/tui/src/tui/theme_picker.rs

@@ -317,7 +317,7 @@ mod tests {
         let mut v = ThemePickerView::new("system".to_string());
         let action = v.handle_key(key(KeyCode::Down));
         assert!(matches!(action, ViewAction::Emit(_)));
-        assert_eq!(selected_name(&action), Some(ThemeId::Whale.name()));
+        assert_eq!(selected_name(&action), Some(ThemeId::Terminal.name()));
     }
 
     #[test]
@@ -337,6 +337,7 @@ mod tests {
         v.handle_key(key(KeyCode::Down));
         v.handle_key(key(KeyCode::Down));
         v.handle_key(key(KeyCode::Down));
+        v.handle_key(key(KeyCode::Down));
         v.handle_key(key(KeyCode::Down)); // -> CatppuccinMocha
         let action = v.handle_key(key(KeyCode::Enter));
         match action {
@@ -376,8 +377,8 @@ mod tests {
     #[test]
     fn digit_jumps_to_row() {
         let mut v = ThemePickerView::new("system".to_string());
-        let action = v.handle_key(key(KeyCode::Char('5')));
-        // Row 5 (1-indexed) -> index 4 -> CatppuccinMocha
+        let action = v.handle_key(key(KeyCode::Char('6')));
+        // Row 6 (1-indexed) -> index 5 -> CatppuccinMocha
         assert_eq!(
             selected_name(&action),
             Some(ThemeId::CatppuccinMocha.name())