set the vert navbar bg color in base.css

web/skins/classic/css/base/skin.css

@@ -662,6 +662,11 @@ margin-left: 0;
 .nav-pills > li > a {
   border-radius: 0;
 }
+
+nav#sidebar.nav {
+  background-color: #f5f5f5; 
+}
+
 .chosen-container {
 text-align: left;
 }

web/skins/classic/views/options.php

@@ -58,7 +58,7 @@ xhtmlHeaders(__FILE__, translate('Options'));
   <?php echo getNavBarHTML(); ?>
   <div class="container-fluid">
     <div class="row flex-nowrap">
-      <nav id="sidebar" style="background-color:#f5f5f5">
+      <nav id="sidebar">
         <ul class="nav nav-pills flex-column">
 <?php
 foreach ( $tabs as $name=>$value ) {