First cut at dark theme

Add reduced brightness background images for main header and lane
buttons. Use CSS filters to reduce brightness of host page images.
Redefine text and background color variables.

public_html/css/hpr.css

@@ -841,3 +841,25 @@ fieldset > table td input[type="radio"] {
 		max-width: 30%;
 	}
 }
+@media (prefers-color-scheme: dark) {
+	:root {
+		--link-primary: #4baabe;
+		--background-primary: #4d4d4d; /* HPR Grey */
+		--text-primary: #fffff7/*#fafafa#fbfbee #dfdfdf */;
+		--shadow-secondary-color: hsl(0, 0%, 40%);
+		--input-border-primary: var(--shadow-secondary-color);
+	}
+	body > header {
+		background-image: url("/images/main-header-background-dark.png");
+	}	
+	a.lane-button {
+		background-image: url("/images/lane-button-background-dark.png");
+	}
+	#hosts td > img {
+		filter: brightness(0.65);
+	}
+	.lane > article > header,
+	#hosts td:nth-child(-n+2) {
+		color: var(--text-primary);
+	}
+}