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

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 {