Use Dylan's DM logo, switch accent to brand blue, add dark mode

Recreate the DM monogram and wordmark as SVG and type so they stay sharp in
both themes, move the accent from violet to the logo's blue, and add a small
theme toggle in the nav that follows the system until the visitor picks one.

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

static/assets/site.js

@@ -3,6 +3,31 @@
   var root = document.documentElement;
   var reduce = window.matchMedia && window.matchMedia('(prefers-reduced-motion: reduce)').matches;
 
+  // Theme toggle: remembers the visitor's choice; otherwise follows the system.
+  var themeBtn = document.getElementById('theme-btn');
+  var mq = window.matchMedia ? window.matchMedia('(prefers-color-scheme: dark)') : null;
+  var current = function () { return root.getAttribute('data-theme') === 'dark' ? 'dark' : 'light'; };
+  var paint = function () {
+    if (!themeBtn) return;
+    var dark = current() === 'dark';
+    themeBtn.setAttribute('aria-label', dark ? 'Switch to light mode' : 'Switch to dark mode');
+    themeBtn.setAttribute('aria-pressed', String(dark));
+  };
+  var apply = function (t, save) {
+    root.setAttribute('data-theme', t);
+    if (save) { try { localStorage.setItem('theme', t); } catch (e) {} }
+    paint();
+  };
+  paint();
+  if (themeBtn) themeBtn.addEventListener('click', function () { apply(current() === 'dark' ? 'light' : 'dark', true); });
+  if (mq && mq.addEventListener) {
+    mq.addEventListener('change', function (e) {
+      var saved = null;
+      try { saved = localStorage.getItem('theme'); } catch (err) {}
+      if (!saved) apply(e.matches ? 'dark' : 'light', false);
+    });
+  }
+
   // Nav border on scroll
   var nav = document.getElementById('nav');
   var onScroll = function () { nav && nav.classList.toggle('is-scrolled', window.scrollY > 8); };