docs: fix custom CSS.

docs/_static/custom.css

@@ -1,7 +1,19 @@
+/* Links in text should be underlined. */
+a { text-decoration: underline; }
+.wy-menu-vertical a, .wy-side-nav-search > a { text-decoration: none; }
+
+/* Match the logo colors in the background. */
+.wy-nav-top, .wy-side-nav-search { background-color: #784b9a; }
+
+/* Make the logo more reasonably sized. */
+.wy-side-nav-search > a img.logo { width: 160px; }
+
 /* Some of our section titles are looong */
-.wy-nav-side, .wy-side-scroll, .wy-menu-vertical { width: 340px; }
-.wy-side-nav-search { width: 325px; }
-.wy-nav-content-wrap { margin-left: 340px; }
+@media screen and (min-width:769px) {
+  .wy-nav-side, .wy-side-scroll, .wy-menu-vertical { width: 340px; }
+  .wy-side-nav-search { width: 325px; }
+  .wy-nav-content-wrap { margin-left: 340px; }
+}
 
 /* We don't have a version picker widget */
 .wy-nav-side { padding-bottom: 0; }