diff --git a/npm_install.py b/npm_install.py
index 7cf7424..86530c2 100644
--- a/npm_install.py
+++ b/npm_install.py
@@ -582,6 +582,53 @@ def install_node_and_yarn(node_pkg: str):
     if not Path("/usr/bin/yarn").exists() and Path("/usr/bin/yarnpkg").exists():
         os.symlink("/usr/bin/yarnpkg","/usr/bin/yarn")
 
+def _prepare_sass(frontend_dir: Path):
+    pj = frontend_dir / "package.json"
+    if not pj.exists():
+        return
+
+    import json, re, os
+    try:
+        data = json.loads(pj.read_text(encoding="utf-8"))
+    except Exception:
+        return
+
+    deps = data.get("dependencies", {}) or {}
+    dev  = data.get("devDependencies", {}) or {}
+    has_node_sass = ("node-sass" in deps) or ("node-sass" in dev)
+    if not has_node_sass:
+        return
+
+    use_dart = (os.environ.get("USE_DART_SASS","").strip() == "1")
+
+    data.setdefault("dependencies", {})
+    data.setdefault("devDependencies", {})
+
+    if use_dart:
+        data["dependencies"].pop("node-sass", None)
+        data["devDependencies"].pop("node-sass", None)
+        if "sass" not in data["dependencies"] and "sass" not in data["devDependencies"]:
+            data["devDependencies"]["sass"] = "^1.77.0"
+        scripts = (data.get("scripts") or {})
+        new_scripts = {}
+        for k, v in scripts.items():
+            new_scripts[k] = re.sub(r"\bnode-sass\b", "sass", v or "")
+        data["scripts"] = new_scripts
+    else:
+        target = "^9.0.0"
+        if "node-sass" in data["dependencies"]:
+            data["dependencies"]["node-sass"] = target
+        else:
+            data["devDependencies"]["node-sass"] = target
+        res = (data.get("resolutions") or {})
+        res["node-gyp"] = "^10.0.0"
+        res["node-sass"] = "^9.0.0"
+        data["resolutions"] = res
+        os.environ["npm_config_node_sass_binary_site"] = "https://github.com/sass/node-sass/releases/download"
+
+    pj.write_text(json.dumps(data, indent=2, ensure_ascii=False) + "\n", encoding="utf-8")
+
+
 def _build_frontend(src_frontend: Path, dest_frontend: Path):
     def _semver(s: str) -> bool:
         return bool(re.match(r"^\d+(?:\.\d+){1,3}$", (s or "").strip()))
@@ -602,6 +649,50 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
                 return ["npx", "-y", "yarn@stable"]
         return None
 
+    def _ensure_yarn_installed():
+        if not shutil.which("npm"):
+            try:
+                apt_try_install(["npm"])
+            except Exception:
+                run(["apt-get", "update"], check=False)
+                run(["apt-get", "install", "-y", "npm"])
+        run(["npm", "install", "-g", "yarn"], check=False)
+
+    yarn_cmd = _pick_yarn_cmd()
+    if not yarn_cmd:
+        _ensure_yarn_installed()
+        yarn_cmd = _pick_yarn_cmd()
+
+    if not yarn_cmd:
+        raise RuntimeError(
+            "Unable to detect or install a valid Yarn. "
+            "Try: apt-get install -y npm && npm i -g yarn."
+        )
+
+    with step("Installing frontend dependencies (yarn)"):
+        os.environ["NODE_ENV"] = "development"
+        os.chdir(src_frontend)
+
+        _prepare_sass(src_frontend)
+
+        cache_dir = (run_out(yarn_cmd + ["cache", "dir"], check=False) or "").strip()
+        if cache_dir and not Path(cache_dir).exists():
+            Path(cache_dir).mkdir(parents=True, exist_ok=True)
+
+        run(yarn_cmd + ["cache", "clean"], check=False)
+        run(yarn_cmd + ["install"])
+
+    with step("Building frontend (yarn build)"):
+        env = os.environ.copy()
+        env["NODE_OPTIONS"] = "--openssl-legacy-provider"
+        run(yarn_cmd + ["build"], env=env)
+
+    with step("Copying frontend artifacts"):
+        shutil.copytree(src_frontend / "dist", dest_frontend, dirs_exist_ok=True)
+        if (src_frontend / "app-images").exists():
+            shutil.copytree(src_frontend / "app-images", dest_frontend / "images", dirs_exist_ok=True)
+
+
     def _ensure_yarn_installed():
         if not shutil.which("npm"):
             try: