fix: a page in the dark color scheme, also one in an iframe, is no longer white while it loads, and test-container.mjs runs the checks with Docker.

test.Dockerfile

@@ -0,0 +1,16 @@
+# The toolkit's checks on Linux: the build runs the typecheck, the lint and the package build, the
+# container the tests; the package stage holds the packed toolkit for the tests of an application.
+
+ARG NODE_VERSION=26
+
+FROM node:${NODE_VERSION}-trixie-slim AS check
+WORKDIR /src
+COPY package.json package-lock.json ./
+RUN npm ci --no-fund --no-audit
+COPY . .
+RUN npm run typecheck && npm run lint && npm run build
+RUN mkdir /pack && npm pack ./dist --pack-destination /pack
+CMD ["npm", "test"]
+
+FROM scratch AS package
+COPY --from=check /pack/ /