Theme pairs with dark mode toggle, gear active state, PALETTE_DEFAULT_DARK (#127)

- Collapse the 10 theme cards into 5 pairs: light swatches top row, dark bottom row
- Dark mode toggle in settings and topbar (sun/moon inline SVG)
- Toggling dark mode switches to the other variant of the selected pair
- Persist pair id in localStorage palette-theme, dark flag in palette-dark
- Head script resolves base+dark to variant; URL ?theme= accepts both ids and wins
- Settings gear highlights like nav tabs on /settings
- PALETTE_DEFAULT_DARK env var sets server default dark state (default on)
- Server-side resolution helpers + tests

internal/web/web.go

@@ -24,6 +24,44 @@ import (
 //go:embed templates/*.html
 var tmplFS embed.FS
 
+// DefaultDark is the server-configured default dark mode state for visitors
+// without stored preferences (#127). Set via PALETTE_DEFAULT_DARK.
+var DefaultDark = true
+
+// SetDefaultDark applies the PALETTE_DEFAULT_DARK env var: unset = dark on,
+// "false"/"0"/"off" = dark off.
+func SetDefaultDark(v string) {
+	switch strings.ToLower(strings.TrimSpace(v)) {
+	case "false", "0", "off":
+		DefaultDark = false
+	default:
+		DefaultDark = true
+	}
+}
+
+// ResolvePreset maps a theme pair id (or explicit variant id) plus dark state
+// to a concrete preset id. Returns the defaults when pair is empty or unknown.
+// Precedence handled by callers: URL ?theme= > stored pair+dark > defaults.
+func ResolvePreset(pair string, dark bool) string {
+	variants := map[string][2]string{ // pair -> {light, dark}
+		"midnight":        {"midnight-light", "midnight"},
+		"smooth":          {"smooth", "smooth-dark"},
+		"pastel-lavender": {"pastel-lavender", "pastel-lavender-dark"},
+		"pastel-peach":    {"pastel-peach", "pastel-peach-dark"},
+		"pastel-cloud":    {"pastel-cloud", "pastel-cloud-dark"},
+	}
+	if v, ok := variants[pair]; ok {
+		if dark {
+			return v[1]
+		}
+		return v[0]
+	}
+	if dark {
+		return "midnight"
+	}
+	return "midnight-light"
+}
+
 //go:embed static
 var staticFS embed.FS
 
@@ -33,8 +71,9 @@ type UI struct {
 
 func New() (*UI, error) {
 	funcs := template.FuncMap{
-		"humanSize": humanSize,
-		"version":   func() string { return Version }, // #93: topbar version label
+		"humanSize":   humanSize,
+		"version":     func() string { return Version },   // #93: topbar version label
+		"defaultDark": func() bool { return DefaultDark }, // #127: server-configured default
 	}
 	t, err := template.New("").Funcs(funcs).ParseFS(tmplFS, "templates/*.html")
 	if err != nil {