feat(theme): --modes light,dark for --pages gallery captures

Sets the bn-theme cookie + emulated prefers-color-scheme before navigation
so each page can be captured in both color modes (Wave B screenshot sets).
Mode-suffixed filenames (NN-<slug>-<mode>.png); empty --modes keeps the
old single site-default capture and naming.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cmd/ninja/cmd/theme.go

@@ -23,7 +23,7 @@ func newThemeCmd() *cobra.Command {
 
 func newThemeScreenshotCmd() *cobra.Command {
 	var gallery, slug, out, mobileOut, themeName, waitSelector string
-	var pages, screenshotsDir string
+	var pages, screenshotsDir, modes string
 	var mobile bool
 	var width, height int
 	cmd := &cobra.Command{
@@ -101,31 +101,44 @@ the rest of the CLI.`,
 			// files the .bnp packer ships under screenshots/ and the registry
 			// ingests into the plugin gallery at publish.
 			if pages != "" {
+				modeList, err := parseModes(modes)
+				if err != nil {
+					return err
+				}
 				if err := os.MkdirAll(screenshotsDir, 0o755); err != nil {
 					return fmt.Errorf("create %s: %w", screenshotsDir, err)
 				}
-				for i, pageSlug := range strings.Split(pages, ",") {
+				n := 0
+				for _, pageSlug := range strings.Split(pages, ",") {
 					pageSlug = strings.TrimSpace(pageSlug)
 					if pageSlug == "" {
 						continue
 					}
-					pURL := shot.PreviewURL(gallery, pageSlug, themeName)
-					fmt.Fprintf(os.Stderr, "capturing page %s: %s\n", pageSlug, pURL)
-					ppng, err := shot.Capture(ctx, shot.Options{
-						URL:          pURL,
-						Width:        width,
-						Height:       height,
-						WaitSelector: waitSelector,
-						Timeout:      45 * time.Second,
-					})
-					if err != nil {
-						return err
+					for _, mode := range modeList {
+						pURL := shot.PreviewURL(gallery, pageSlug, themeName)
+						fmt.Fprintf(os.Stderr, "capturing page %s (%s): %s\n", pageSlug, modeLabel(mode), pURL)
+						ppng, err := shot.Capture(ctx, shot.Options{
+							URL:          pURL,
+							Width:        width,
+							Height:       height,
+							WaitSelector: waitSelector,
+							Timeout:      45 * time.Second,
+							Mode:         mode,
+						})
+						if err != nil {
+							return err
+						}
+						n++
+						name := fmt.Sprintf("%02d-%s.png", n, slugFileName(pageSlug))
+						if mode != "" {
+							name = fmt.Sprintf("%02d-%s-%s.png", n, slugFileName(pageSlug), mode)
+						}
+						dest := filepath.Join(screenshotsDir, name)
+						if err := os.WriteFile(dest, ppng, 0o644); err != nil {
+							return fmt.Errorf("write %s: %w", dest, err)
+						}
+						fmt.Printf("wrote %s (%d bytes)\n", dest, len(ppng))
 					}
-					dest := filepath.Join(screenshotsDir, fmt.Sprintf("%02d-%s.png", i+1, slugFileName(pageSlug)))
-					if err := os.WriteFile(dest, ppng, 0o644); err != nil {
-						return fmt.Errorf("write %s: %w", dest, err)
-					}
-					fmt.Printf("wrote %s (%d bytes)\n", dest, len(ppng))
 				}
 			}
 			return nil
@@ -142,9 +155,34 @@ the rest of the CLI.`,
 	cmd.Flags().IntVar(&height, "height", 900, "Desktop viewport height")
 	cmd.Flags().StringVar(&pages, "pages", "", "Comma-separated page slugs to capture into --screenshots-dir (e.g. /,/blog,/about)")
 	cmd.Flags().StringVar(&screenshotsDir, "screenshots-dir", "screenshots", "Directory for --pages captures (packed into the .bnp)")
+	cmd.Flags().StringVar(&modes, "modes", "", `Comma-separated color modes for --pages captures: "light", "dark" or "light,dark" (sets the bn-theme cookie per capture; empty = site default, unsuffixed filenames)`)
 	return cmd
 }
 
+// parseModes validates the --modes flag: empty means one site-default capture
+// (mode ""), otherwise each entry must be "light" or "dark".
+func parseModes(modes string) ([]string, error) {
+	if strings.TrimSpace(modes) == "" {
+		return []string{""}, nil
+	}
+	var out []string
+	for _, m := range strings.Split(modes, ",") {
+		m = strings.TrimSpace(m)
+		if m != "light" && m != "dark" {
+			return nil, fmt.Errorf("--modes: %q is not a valid mode (want light or dark)", m)
+		}
+		out = append(out, m)
+	}
+	return out, nil
+}
+
+func modeLabel(mode string) string {
+	if mode == "" {
+		return "default"
+	}
+	return mode
+}
+
 // slugFileName turns a page slug into a filename fragment ("/" → "home",
 // "/blog/post" → "blog-post").
 func slugFileName(slug string) string {

internal/shot/shot.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/chromedp/cdproto/emulation"
+	"github.com/chromedp/cdproto/network"
 	"github.com/chromedp/chromedp"
 )
 
@@ -22,6 +24,7 @@ type Options struct {
 	FullPage     bool          // capture the full scroll height, not just the viewport
 	WaitSelector string        // CSS selector to wait for before capturing (e.g. "section")
 	Timeout      time.Duration // overall navigation+capture timeout
+	Mode         string        // "" (site default), "light" or "dark" — sets the bn-theme cookie + emulated prefers-color-scheme before navigation
 }
 
 // PreviewURL composes the gallery-page URL with the render-only override.
@@ -67,8 +70,25 @@ func Capture(ctx context.Context, opts Options) ([]byte, error) {
 	var buf []byte
 	tasks := chromedp.Tasks{
 		chromedp.EmulateViewport(int64(opts.Width), int64(opts.Height)),
-		chromedp.Navigate(opts.URL),
 	}
+	if opts.Mode == "light" || opts.Mode == "dark" {
+		u, err := url.Parse(opts.URL)
+		if err != nil {
+			return nil, fmt.Errorf("parse %s: %w", opts.URL, err)
+		}
+		host := u.Hostname()
+		mode := opts.Mode
+		tasks = append(tasks,
+			chromedp.ActionFunc(func(ctx context.Context) error {
+				return network.SetCookie("bn-theme", mode).
+					WithDomain(host).WithPath("/").Do(ctx)
+			}),
+			emulation.SetEmulatedMedia().WithFeatures([]*emulation.MediaFeature{
+				{Name: "prefers-color-scheme", Value: mode},
+			}),
+		)
+	}
+	tasks = append(tasks, chromedp.Navigate(opts.URL))
 	if opts.WaitSelector != "" {
 		tasks = append(tasks, chromedp.WaitVisible(opts.WaitSelector, chromedp.ByQuery))
 	} else {