Add theme switchers: Admin UI (A/B/C) + Display (5 themes)

Admin UI: Dark Minimal / Retro Terminal / Warm Editorial via header buttons
Display: Classic White / Dark / Sepia / Nord / Terminal via Settings tab
Both switchers persist via localStorage / config.json

admin.py

@@ -337,6 +337,58 @@ def api_layout_pack():
     return jsonify({"ok": True, "items": cfg["layout"]["items"]})
 
 
+DISPLAY_THEMES = {
+    "default": {
+        "label": "Classic White",
+        "bg": "#ffffff", "fg": "#000000",
+        "accent": "#ff8800",
+    },
+    "dark": {
+        "label": "Dark Mode",
+        "bg": "#111111", "fg": "#f0f0f0",
+        "accent": "#6d8eff",
+    },
+    "sepia": {
+        "label": "Sepia",
+        "bg": "#f4ecd8", "fg": "#5b4636",
+        "accent": "#c0392b",
+    },
+    "nord": {
+        "label": "Nord",
+        "bg": "#eceff4", "fg": "#2e3440",
+        "accent": "#88c0d0",
+    },
+    "terminal": {
+        "label": "Terminal (Green)",
+        "bg": "#0d0d00", "fg": "#e8c840",
+        "accent": "#ffcc00",
+    },
+}
+
+
+@app.route("/api/display_theme", methods=["GET", "POST"])
+def api_display_theme():
+    """Theme für das Display. GET = aktuelles Theme, POST = setzen."""
+    a = require_auth()
+    if a: return a
+    cfg = dashboard_mod.load_config()
+    if request.method == "GET":
+        theme_key = cfg.get("display_theme", "default")
+        theme = DISPLAY_THEMES.get(theme_key, DISPLAY_THEMES["default"])
+        return jsonify({
+            "current": theme_key,
+            "themes": {k: v["label"] for k, v in DISPLAY_THEMES.items()},
+            "theme": theme,
+        })
+    data = request.get_json(silent=True) or {}
+    theme_key = data.get("theme", "default")
+    if theme_key not in DISPLAY_THEMES:
+        return jsonify({"ok": False, "error": "unknown theme"}), 400
+    cfg["display_theme"] = theme_key
+    dashboard_mod.save_config(cfg)
+    return jsonify({"ok": True, "current": theme_key, "theme": DISPLAY_THEMES[theme_key]})
+
+
 @app.route("/api/plugin_config/<plugin_name>", methods=["GET", "POST"])
 def api_plugin_config(plugin_name):
     """Plugin-spezifische Config (separat vom Layout)."""