Use CSS media block for dark theme

Moved CSS dark theme rules into a media dark theme to automatically use
it if the user device is configured to prioritize dark themes.

src/en/assets/luca0n.css

@@ -144,6 +144,7 @@ p {
 /**************/
 /* Dark theme */
 /**************/
+@media (prefers-color-scheme: dark) {
 	body {
 		background-color: #222;
 		color: #ddd;
@@ -178,6 +179,7 @@ header a:visited {
 		color: black;
 		background: linear-gradient(to top, rgb(238, 112, 0), rgb(255, 178, 30));
 	}
+}
 
 @media screen and (max-width: 768px) {
 	#menu-default {