Add Crimson theme: red on OLED black

Pure #000 background (OLED pixels off), near-black panels with dark red
borders, and a red accent (#e0263a) that passes 4.5:1 as text on black and
behind white button labels. Badges use a brighter red to stay legible on
their tinted background.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/static/css/themes.css

@@ -164,6 +164,28 @@
   --shadow: 0 10px 40px rgba(255, 79, 216, 0.12);
 }
 
+/* Red on true black: #000 backgrounds switch OLED pixels off. The accent
+   passes 4.5:1 both as text on black and behind white button labels. */
+[data-theme="crimson"] {
+  color-scheme: dark;
+  --bg: #000000;
+  --surface: #0a0a0a;
+  --surface-2: #151011;
+  --border: #2e1418;
+  --text: #f5f1f1;
+  --muted: #b5a9ab;
+  --faint: #75676a;
+  --accent: #e0263a;
+  --accent-text: #ffffff;
+  --accent-soft: rgba(224, 38, 58, 0.16);
+  --danger: #ff5c6c;
+  --success: #4fd1a1;
+  --warn: #f5b84b;
+  --shadow: 0 10px 30px rgba(0, 0, 0, 0.7);
+}
+/* Small badge text on the red tint needs a brighter red to stay legible. */
+[data-theme="crimson"] .badge:not(.badge--warn):not(.badge--muted) { color: #ff5a6a; }
+
 [data-theme="daylight"] {
   color-scheme: light;
   --bg: #f6f7f9;