feat(tui): implement dark mode with runtime /theme switching

- Paint dark background at app shell level via appStyle() helper
- Add /theme slash command for runtime light/dark switching
- Add comprehensive dark-mode style tests
- Document --theme dark in README Quick Start

README.md

@@ -23,6 +23,12 @@ export AGENTU_API_KEY='your-api-key'
 go run .
 ```
 
+Use dark mode when your terminal is dark:
+
+```sh
+go run . --theme dark
+```
+
 Use `--yolo` only when you want agentu to write files or run shell commands:
 
 ```sh

internal/tui/app.go

@@ -145,7 +145,7 @@ var slashCommands = []slashCommand{
 	{Name: "/new", Description: "new session"},
 	{Name: "/status", Description: "show changed files"},
 	{Name: "/diff", Description: "show unstaged diff"},
-	{Name: "/test", Description: "run project tests"},
+	{Name: "/theme", Description: "switch theme light|dark"},
 }
 
 func newModel(ctx context.Context, assistant *agent.Agent, opts Options) model {
@@ -389,7 +389,7 @@ func (m *model) submit() (tea.Model, tea.Cmd) {
 		m.status = "ready"
 		m.refreshViewport(true)
 		return *m, nil
-	case "/model", "/compact":
+	case "/model", "/compact", "/theme":
 		return m.runLocalCommand(input)
 	}
 
@@ -1090,6 +1090,9 @@ func fitLine(text string, width int) string {
 }
 
 func (m *model) runLocalCommand(input string) (tea.Model, tea.Cmd) {
+	if strings.HasPrefix(input, "/theme") {
+		return m.handleThemeCommand(input)
+	}
 	output, err := m.handleLocalCommand(input)
 	if err == errOpenPicker {
 		m.openSessionPicker()
@@ -1114,6 +1117,35 @@ func (m *model) runLocalCommand(input string) (tea.Model, tea.Cmd) {
 	return *m, nil
 }
 
+func (m *model) handleThemeCommand(input string) (tea.Model, tea.Cmd) {
+	fields := strings.Fields(input)
+	if len(fields) < 2 {
+		m.input.SetValue("")
+		m.afterInputChanged()
+		m.messages = append(m.messages, message{
+			role: roleSystem, content: "theme: " + string(m.theme.Mode) + "\nusage: /theme light|dark",
+		})
+		m.refreshViewport(true)
+		return *m, nil
+	}
+	mode, err := ParseThemeMode(fields[1])
+	if err != nil {
+		m.input.SetValue("")
+		m.afterInputChanged()
+		m.messages = append(m.messages, message{role: roleError, content: err.Error()})
+		m.refreshViewport(true)
+		return *m, nil
+	}
+	m.theme = themeForMode(mode)
+	m.styles = m.theme.styles()
+	applyTextareaTheme(&m.input, m.theme)
+	m.input.SetValue("")
+	m.afterInputChanged()
+	m.messages = append(m.messages, message{role: roleSystem, content: "theme: " + string(mode)})
+	m.refreshViewport(true)
+	return *m, nil
+}
+
 func (m model) handleLocalCommand(input string) (string, error) {
 	fields := strings.Fields(input)
 	if len(fields) == 0 {

internal/tui/app_test.go

@@ -604,6 +604,115 @@ func TestParseThemeMode(t *testing.T) {
 	}
 }
 
+func requireStyleColor(t *testing.T, name string, got lipgloss.TerminalColor, want string) {
+	t.Helper()
+	color, ok := got.(lipgloss.Color)
+	if !ok {
+		t.Fatalf("%s = %T, want lipgloss.Color", name, got)
+	}
+	if string(color) != want {
+		t.Fatalf("%s = %q, want %q", name, string(color), want)
+	}
+}
+
+func requireNoStyleColor(t *testing.T, name string, got lipgloss.TerminalColor) {
+	t.Helper()
+	if _, ok := got.(lipgloss.NoColor); !ok {
+		t.Fatalf("%s should not paint a color, got %T", name, got)
+	}
+}
+
+func TestDarkThemePaintsAppBackground(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model", ThemeMode: ThemeDark})
+	next, _ := m.Update(tea.WindowSizeMsg{Width: 100, Height: 30})
+	m = next.(model)
+	rendered := stripANSI(m.View())
+	if !strings.Contains(rendered, "theme dark") {
+		t.Fatalf("rendered view missing 'theme dark':\n%s", rendered)
+	}
+	if m.theme.Mode != ThemeDark {
+		t.Fatalf("theme.Mode = %q, want %q", m.theme.Mode, ThemeDark)
+	}
+	want := darkTheme()
+	requireStyleColor(t, "App.Background", m.styles.App.GetBackground(), want.Background)
+	requireStyleColor(t, "App.Foreground", m.styles.App.GetForeground(), want.Text)
+}
+
+func TestDarkThemeKeepsComponentBackgroundsLightweight(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model", ThemeMode: ThemeDark})
+	next, _ := m.Update(tea.WindowSizeMsg{Width: 100, Height: 30})
+	m = next.(model)
+	requireNoStyleColor(t, "Viewport.Background", m.styles.Viewport.GetBackground())
+	requireNoStyleColor(t, "InputBox.Background", m.styles.InputBox.GetBackground())
+	requireNoStyleColor(t, "AssistantMsg.Background", m.styles.AssistantMsg.GetBackground())
+	requireNoStyleColor(t, "Activity.Background", m.styles.Activity.GetBackground())
+	m.running = true
+	m.status = "thinking..."
+	m.resize()
+	if backgroundPattern.MatchString(m.activityView()) {
+		t.Fatalf("activity view should not paint a background:\n%q", m.activityView())
+	}
+}
+
+func TestDarkThemeStylesUseDarkPalette(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model", ThemeMode: ThemeDark})
+	want := darkTheme()
+	requireStyleColor(t, "Header.Foreground", m.styles.Header.GetForeground(), want.Title)
+	requireStyleColor(t, "Muted.Foreground", m.styles.Muted.GetForeground(), want.Muted)
+	requireStyleColor(t, "Pill.Background", m.styles.Pill.GetBackground(), want.SurfaceAlt)
+	requireStyleColor(t, "input.FocusedStyle.Base.Foreground", m.input.FocusedStyle.Base.GetForeground(), want.Text)
+	requireStyleColor(t, "input.FocusedStyle.Placeholder.Foreground", m.input.FocusedStyle.Placeholder.GetForeground(), want.Muted)
+}
+
+func TestModelCommandShowsDarkTheme(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model", ThemeMode: ThemeDark})
+	m.input.SetValue("/model")
+	next, _ := m.submit()
+	rendered := next.(model).renderMessages()
+	if !strings.Contains(rendered, "theme: dark") {
+		t.Fatalf("model command output missing 'theme: dark':\n%s", rendered)
+	}
+}
+
+func TestThemeSlashCommandSwitchesAtRuntime(t *testing.T) {
+	m := newModel(context.Background(), nil, Options{ModelName: "test-model"})
+	if m.theme.Mode != ThemeLight {
+		t.Fatalf("initial theme = %q, want light", m.theme.Mode)
+	}
+	requireNoStyleColor(t, "initial App.Background", m.styles.App.GetBackground())
+
+	// Switch to dark
+	m.input.SetValue("/theme dark")
+	next, _ := m.submit()
+	m = next.(model)
+	if m.theme.Mode != ThemeDark {
+		t.Fatalf("after /theme dark: Mode = %q, want dark", m.theme.Mode)
+	}
+	requireStyleColor(t, "dark App.Background", m.styles.App.GetBackground(), darkTheme().Background)
+	last := m.messages[len(m.messages)-1]
+	if !strings.Contains(last.content, "theme: dark") {
+		t.Fatalf("expected confirmation message, got %q", last.content)
+	}
+
+	// Switch back to light
+	m.input.SetValue("/theme light")
+	next, _ = m.submit()
+	m = next.(model)
+	if m.theme.Mode != ThemeLight {
+		t.Fatalf("after /theme light: Mode = %q, want light", m.theme.Mode)
+	}
+	requireNoStyleColor(t, "restored App.Background", m.styles.App.GetBackground())
+
+	// No arg shows usage
+	m.input.SetValue("/theme")
+	next, _ = m.submit()
+	m = next.(model)
+	last = m.messages[len(m.messages)-1]
+	if !strings.Contains(last.content, "usage: /theme") {
+		t.Fatalf("expected usage hint, got %q", last.content)
+	}
+}
+
 func TestCleanToolLog(t *testing.T) {
 	got := cleanToolLog("\n[tool] shell_run {\"command\":\"pwd\"}\n")
 	want := `shell_run {"command":"pwd"}`

internal/tui/theme.go

@@ -130,10 +130,17 @@ func darkTheme() theme {
 	}
 }
 
+func appStyle(t theme) lipgloss.Style {
+	style := lipgloss.NewStyle().Foreground(lipgloss.Color(t.Text))
+	if t.Mode == ThemeDark {
+		style = style.Background(lipgloss.Color(t.Background))
+	}
+	return style
+}
+
 func (t theme) styles() styles {
 	return styles{
-		App: lipgloss.NewStyle().
+		App: appStyle(t),
-			Foreground(lipgloss.Color(t.Text)),
 
 		Muted: lipgloss.NewStyle().
 			Foreground(lipgloss.Color(t.Muted)),