cmd/pkgserver: fix dark mode in test web UI

cmd/pkgserver/ui/static/test.scss

@@ -1,3 +1,19 @@
+:root {
+    --bg: #d3d3d3;
+    --fg: black;
+}
+
+@media (prefers-color-scheme: dark) {
+    :root {
+        --bg: #2c2c2c;
+        --fg: ghostwhite;
+    }
+}
+
+html {
+    background-color: var(--bg);
+    color: var(--fg);
+}
 h1, p, summary {
     font-family: sans-serif;
 }
@@ -9,7 +25,7 @@ h1, p, summary {
 details.test-node {
     margin-left: 1rem;
     padding: 0.2rem 0.5rem;
-    border-left: 2px dashed black;
+    border-left: 2px dashed var(--fg);
     > summary {
         cursor: pointer;
     }
@@ -27,7 +43,7 @@ details.test-node {
          * [2]: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/summary#default_style
          * [3]: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/summary#changing_the_summarys_icon
          */
-        color: black;
+        color: var(--fg);
         content: url("/static/success-closed.svg") / "success";
     }
     &.success[open] > summary::marker {