docs: html: improve css for small displays

fixed-width navi/toc links were not switched in color for small width
displays, and thus they were barely readable as the background
switches to dark for small widths.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/custom.css

@@ -32,3 +32,11 @@ div.sphinxsidebar form.search {
 div.sphinxsidebar h3 {
     width: 100%;
 }
+@media screen and (max-width: 875px) {
+    div.sphinxsidebar span {
+        color: #AAA;
+    }
+    ul li.toctree-l1 > a {
+        color: #FFF;
+    }
+}