cmd/pkgserver: fix dark mode in test web UI

cmd/pkgserver/ui/static/_test_common.scss

@@ -0,0 +1,62 @@
+html {
+    background-color: var(--bg);
+    color: var(--fg);
+}
+
+h1, p, summary {
+    font-family: sans-serif;
+}
+
+.root {
+    margin: 1rem 0;
+}
+
+details.test-node {
+    margin-left: 1rem;
+    padding: 0.2rem 0.5rem;
+    border-left: 2px dashed var(--fg);
+    > summary {
+        cursor: pointer;
+    }
+    &.success > summary::marker {
+        /*
+         * WebKit only supports color and font-size properties in ::marker, and
+         * its ::-webkit-details-marker doesn't seem to work whatsoever; thus,
+         * set a color as a fallback: while it may be confusing for colorblind
+         * individuals, it's better than no indication of a test's state.
+         * See: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::marker#browser_compatibility
+         */
+        color: var(--fg);
+        /* content set in theme-specific file. */
+    }
+    &.failure > summary::marker {
+        color: red;
+        content: url("/static/failure-closed.svg");
+    }
+    &.failure[open] > summary::marker {
+        content: url("/static/failure-open.svg");
+    }
+    &.skip > summary::marker {
+        color: blue;
+        content: url("/static/skip-closed.svg");
+    }
+    &.skip[open] > summary::marker {
+        content: url("/static/skip-open.svg");
+    }
+}
+
+p.test-desc {
+    margin: 0 0 0 1rem;
+    padding: 2px 0;
+    > pre {
+        margin: 0;
+    }
+}
+
+.hidden {
+    display: none;
+}
+
+.italic {
+    font-style: italic;
+}