Update CSS imports and Dockerfile for consistent styles handling

- Enforced `!important` for `--color-background` in `variables.css`.
- Centralized CSS imports in `Layout.astro` for cleaner markup.
- Adjusted Dockerfile to ensure CSS variables are copied to `public/styles`.
- Refined fallback handling in `index.css` for `background-color`.

Dockerfile

@@ -4,6 +4,9 @@ WORKDIR /app
 COPY package*.json ./
 RUN npm ci
 COPY . .
+# Ensure CSS variables are present
+RUN mkdir -p public/styles
+RUN cp -r styles/* public/styles/ || true
 RUN npm run build
 
 FROM node:20-alpine AS production