diff --git a/AGENTS.md b/AGENTS.md
index a1da21b..5a5dc3c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -194,5 +194,9 @@ except `public/cache/*` and `novaconium/contact-log.txt`.
   `App/css/main.css` (project-owned; docker-compose mounts `App/css` to
   the container's `public/css`, and it's served at `/css/main.css`):
   `sass --load-path=App/sass --load-path=novaconium/sass/defaults --no-source-map novaconium/sass/main.sass App/css/main.css`
-  — commit the regenerated CSS. See `/admin/docs/styling` for a Docker
-  fallback if `sass` isn't installed locally.
+  — commit the regenerated CSS. The Docker image ships Dart Sass standalone
+  (pinned `DART_SASS_VERSION` in the `Dockerfile`, symlinked to
+  `/usr/local/bin/sass`), so the same command runs inside the container;
+  bump that version deliberately, like the PHP base tag. See
+  `/admin/docs/styling` for the Docker fallback if `sass` isn't installed
+  locally.
diff --git a/Dockerfile b/Dockerfile
index 753c73d..4ea7846 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,4 @@
 # Official PHP + Apache image for running novaconium in production.
-# See /admin/docs/docker for the bind-mounted paths, docker-entrypoint.sh's
-# seeding/permissions behavior, and optional MySQL wiring.
 
 # Build: docker build --no-cache -t novaconium:latest .
 
@@ -14,11 +12,31 @@ FROM php:8.5.8-apache-trixie
 # rebuild.
 
 RUN apt-get update \
-    && apt-get install -y --no-install-recommends libsqlite3-dev \
+    && apt-get install -y --no-install-recommends libsqlite3-dev curl ca-certificates \
     && rm -rf /var/lib/apt/lists/* \
     && docker-php-ext-install pdo_sqlite pdo_mysql \
     && a2enmod rewrite
 
+# Dart Sass standalone, so the container can compile
+# novaconium/sass/main.sass -> App/css/main.css itself (see AGENTS.md
+# "Conventions worth knowing" and /admin/docs/styling). Debian trixie has
+# no usable dart-sass package, so pull the pinned upstream release tarball.
+# Bump DART_SASS_VERSION deliberately, like the PHP base tag above.
+ENV DART_SASS_VERSION=1.83.4
+RUN set -eux; \
+    arch="$(dpkg --print-architecture)"; \
+    case "$arch" in \
+      amd64) sass_arch='x64' ;; \
+      arm64) sass_arch='arm64' ;; \
+      *) echo "unsupported arch: $arch" >&2; exit 1 ;; \
+    esac; \
+    curl -fsSL -o /tmp/dart-sass.tar.gz \
+      "https://github.com/sass/dart-sass/releases/download/${DART_SASS_VERSION}/dart-sass-${DART_SASS_VERSION}-linux-${sass_arch}.tar.gz"; \
+    tar -xzf /tmp/dart-sass.tar.gz -C /opt; \
+    rm /tmp/dart-sass.tar.gz; \
+    ln -s /opt/dart-sass/sass /usr/local/bin/sass; \
+    sass --version
+
 # Point DocumentRoot at public/ and allow .htaccess overrides there.
 RUN sed -ri -e 's#/var/www/html#/var/www/html/public#g' \
       /etc/apache2/sites-available/*.conf \