diff --git a/soapbox/app.py b/soapbox/app.py
index cdbe15a..3a156e5 100644
--- a/soapbox/app.py
+++ b/soapbox/app.py
@@ -109,8 +109,9 @@ def _serve_public(cfg: Config, path: str):
             return redirect(request.path + "/", code=301)
         path = f"{path}index.html" if path else "index.html"
     resp = send_from_directory(root, path)
-    if path.startswith("theme/"):
-        # URL-en inneholder innholdshash (?v=), så filene kan caches lenge
+    if path.startswith("theme/") and request.args.get("v"):
+        # URL-en inneholder innholdshash (?v=), så filen kan caches «for alltid».
+        # Uten hash (f.eks. url("favicon.svg") inne i CSS) gjelder vanlig cache.
         resp.headers["Cache-Control"] = "public, max-age=31536000, immutable"
     elif "." in os.path.basename(path) and not path.endswith(".html"):
         resp.headers["Cache-Control"] = "public, max-age=3600"
diff --git a/soapbox/build.py b/soapbox/build.py
index 8f814d3..d170549 100644
--- a/soapbox/build.py
+++ b/soapbox/build.py
@@ -5,9 +5,10 @@ Utdata er rene filer som kan serveres av hva som helst – appen selv, Caddy
 eller en CDN.
 
 Alle lenker i genererte sider er relative. Malene får en ``rel()``-funksjon
-som regner ut stien fra *denne* siden til et mål, så ``rel("theme/style.css")``
-blir ``theme/style.css`` på forsiden og ``../../theme/style.css`` i et
-gjesteinnlegg. Dermed kan hele ``public/`` flyttes til hvilken som helst
+som regner ut stien fra *denne* siden til et mål, så ``rel("post2/")`` blir
+``post2/`` på forsiden og ``../../post2/`` i et gjesteinnlegg. Temafiler
+refereres med ``asset("theme/style.css")``, som i tillegg legger på en
+innholdshash (``?v=…``) for cache-busting. Dermed kan hele ``public/`` flyttes til hvilken som helst
 sub-path uten rebuild. Unntaket er ``feed.xml``, der RSS krever absolutte
 URL-er; den bruker ``SOAPBOX_SITE_URL``.
 """
@@ -71,22 +72,24 @@ def build_site(cfg: Config) -> int:
     for d in reversed(cfg.theme_dirs):
         if (d / "static").is_dir():
             shutil.copytree(d / "static", tmp / "theme", dirs_exist_ok=True)
-    versions = (
-        {
-            f"theme/{f.relative_to(tmp / 'theme').as_posix()}": hashlib.sha256(
-                f.read_bytes()
-            ).hexdigest()[:8]
-            for f in (tmp / "theme").rglob("*")
-            if f.is_file()
-        }
-        if (tmp / "theme").is_dir()
-        else {}
-    )
+    versions = {
+        f"theme/{f.relative_to(tmp / 'theme').as_posix()}": hashlib.sha256(
+            f.read_bytes()
+        ).hexdigest()[:8]
+        for f in (tmp / "theme").rglob("*")
+        if f.is_file()
+    }
 
     def render(template: str, page_path: str, **ctx) -> str:
+        def rel(target: str) -> str:
+            return relative_to(page_path, target)
+
         def asset(target: str) -> str:
-            url = relative_to(page_path, target)
-            return f"{url}?v={versions[target]}" if target in versions else url
+            # Feil raskt: en skrivefeil i temaet skal synes ved lagring, ikke
+            # ende som en 404 som er cachet i et år.
+            if target not in versions:
+                raise ValueError(f"asset(): fant ikke temafilen {target!r}")
+            return f"{rel(target)}?v={versions[target]}"
 
         return env.get_template(template).render(
             site=site,
@@ -94,7 +97,7 @@ def build_site(cfg: Config) -> int:
             source_url=SOURCE_URL,
             site_url=cfg.site_url,  # absolutt; kun for Open Graph/canonical
             page_path=page_path,
-            rel=lambda target: relative_to(page_path, target),
+            rel=rel,
             now=datetime.now(UTC),
             **ctx,
         )
diff --git a/tests/test_app.py b/tests/test_app.py
index ad1ea11..74562b3 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -65,6 +65,8 @@ def test_full_flow_owner(client, cfg):
     assert re.search(r'href="\.\./theme/favicon\.svg\?v=[0-9a-f]{8}"', html)
     r = client.get(f"/theme/style.css?v={m.group(1)}")
     assert r.status_code == 200 and "immutable" in r.headers["Cache-Control"]
+    # uten hash (f.eks. url("favicon.svg") fra CSS) skal cachen være kort
+    assert "immutable" not in client.get("/theme/favicon.svg").headers["Cache-Control"]
     assert 'href="../"' in html  # intern lenke omskrevet til relativ
     assert client.get("/grønt-er-skjønt/mitt-bilde.png").status_code == 200
     front = client.get("/").text
@@ -108,7 +110,7 @@ def test_guest_flow_and_isolation(client, cfg):
     assert (cfg.content_dir / "guests/gjest1/post-1/index.md").exists()
 
     html = client.get("/gjest1/post-1/").text
-    assert re.search(r'href="\.\./\.\./theme/style\.css\?v=', html)
+    assert re.search(r'href="\.\./\.\./theme/style\.css\?v=[0-9a-f]{8}"', html)
     assert "Gjest En" in html
     assert "Post 1" in client.get("/gjest1/").text
     assert "Post 1" in client.get("/").text  # gjesteinnlegg vises også på forsiden
@@ -149,7 +151,7 @@ def test_subpath_mount(tmp_path):
     r = client.post("/blog/admin/new", data={"csrf": csrf(client, "/blog")})
     assert r.headers["Location"].startswith("/blog/admin/edit/")
     html = client.get("/blog/").text
-    assert re.search(r'href="theme/style\.css\?v=', html)
+    assert re.search(r'href="theme/style\.css\?v=[0-9a-f]{8}"', html)
     assert re.search(r'href="admin/"', html)
     assert client.get("/blog/theme/style.css").status_code == 200
 
@@ -443,3 +445,14 @@ def test_asset_hash_changes_with_content(client, cfg):
     client.post("/admin/settings", data={"csrf": token, "title": "T", "theme": "green2"})
     after = re.search(r"style\.css\?v=([0-9a-f]{8})", client.get("/").text).group(1)
     assert before != after
+
+
+def test_asset_unknown_file_fails_build(client, cfg):
+    theme = cfg.content_dir / "themes" / "feil" / "templates"
+    theme.mkdir(parents=True)
+    (theme / "index.html").write_text("{{ asset('theme/finnes-ikke.css') }}")
+    login(client)
+    import pytest
+
+    with pytest.raises(ValueError, match="finnes-ikke"):
+        client.post("/admin/settings", data={"csrf": csrf(client), "title": "T", "theme": "feil"})