Add a light mode to the site.

default_environment/stylesheet/main.css

@@ -5,6 +5,7 @@
   --header-divider-color: #6a687a;
 
   --stream-divider-color: #6ccff6;
+  --stream-post-background-color: #1f1f1f;
 
   --src-font-family: ui-monospace, "Cascadia Code", "Source Code Pro", Menlo,
     Consolas, "DejaVu Sans Mono", monospace;
@@ -21,6 +22,28 @@
   --table-even-background-color: #141414;
 }
 
+@media (prefers-color-scheme: light) {
+  :root {
+    --site-background-color: #f5f5f5;
+    --site-text-color: #000003;
+    --header-divider-color: #959785;
+
+    --stream-divider-color: #933009;
+    --stream-post-background-color: #e0e0e0;
+
+    --src-block-background-color: #ebebeb;
+    --src-block-border-color: #7b7d70;
+    --src-block-language-color: #f5f5f5;
+    --src-block-language-background: #7b7d70;
+
+    --quote-block-border-color: #7b7d70;
+
+    --table-border-color: #959785;
+    --table-odd-background-color: #f5f5f5;
+    --table-even-background-color: #ebebeb;
+  }
+}
+
 body {
   color: var(--site-text-color);
   background-color: var(--site-background-color);
@@ -76,7 +99,7 @@ body {
 
   /* A blog post in a blog stream (for example, the homepage). */
   .blog_stream_post {
-    background: #1f1f1f;
+    background: var(--stream-post-background-color);
     padding: 1rem 0.2rem;
   }