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`.

styles/variables.css

@@ -1,6 +1,6 @@
 :root {
 	/* Colors */
-	--color-background: #000;
+	--color-background: #000000 !important;
 	--color-text: #f5f5f5;
 	--color-accent-green: #213b28;
 	--color-accent-beige: #ceb39b;