cmd/pkgserver: fix dark mode in test web UI

cmd/pkgserver/ui/static/test_dark.scss

@@ -0,0 +1,19 @@
+@use "test_common";
+
+:root {
+    --bg: #2c2c2c;
+    --fg: ghostwhite;
+}
+
+/*
+ * The color of an SVG in a content: cannot be controlled via CSS as it's not
+ * present in the DOM, so use duplicated versions.
+ */
+details.test-node.success {
+    > summary::marker {
+        content: url("/static/success-closed-white.svg");
+    }
+    &[open] > summary::marker {
+        content: url("/static/success-open-white.svg");
+    }
+}