docs: match sidebar background color to logo.

Author: whitequark

docs/_static/custom.css

@@ -2,6 +2,9 @@
 a { text-decoration: underline; }
 .wy-menu-vertical a, .wy-side-nav-search > a { text-decoration: none; }
 
+/* Match the logo colors in the background. */
+.wy-side-nav-search { background-color: #784b9a; }
+
 /* Make the logo more reasonably sized. */
 .wy-side-nav-search > a img.logo { width: 200px; }