Integrate light theme's `menu-active-bg-color` using a media query

public/css/icinga/base.less

@@ -63,6 +63,7 @@
 @menu-hover-bg-color: fade(@menu-bg-color, 50%);
 @menu-search-hover-bg-color: darken(@menu-bg-color, 20%);
 @menu-active-bg-color: #181742;
+@menu-active-hover-bg-color: darken(@menu-active-bg-color, 20%);
 @menu-color: #DBDBDB;
 @menu-active-color: @text-color;
 @menu-highlight-color: @icinga-blue;
@@ -108,10 +109,11 @@
     --tab-hover-bg-color: fade(#F5F9FA, 50%);
 
     --menu-color: #676767;
-
     --menu-bg-color: #DEECF1;
     --menu-hover-bg-color: fade(#DEECF1, 50%);
     --menu-search-hover-bg-color: darken(#DEECF1, 20%);
+    --menu-active-bg-color: #EDF7FC;
+    --menu-active-hover-bg-color: darken(#EDF7FC, 20%);
 
     --text-color: #535353;
     --text-color-light: fade(#535353, 75%);

public/css/icinga/menu.less

@@ -74,7 +74,7 @@
       display: none;
     }
 
-    background-color: @menu-active-bg-color;
+    .var(background-color, menu-active-bg-color);
   }
 
   &.no-icon > a {
@@ -87,7 +87,7 @@
 
   &.active:not(.selected) > a:focus,
   &.active:not(.selected) > a:hover {
-    background-color: darken(@menu-active-bg-color, 20);
+    .var(background-color, menu-active-hover-bg-color);
   }
 
   &:not(.selected) > a:hover,
@@ -217,7 +217,7 @@
   width: 100%;
 
   &.active {
-    background-color: @menu-active-bg-color;
+    .var(background-color, menu-active-bg-color);
   }
 
   &:hover,
@@ -384,7 +384,7 @@
 }
 
 .search-input:focus ~ .search-reset:hover {
-  background-color: darken(@menu-active-bg-color, 20);
+  .var(background-color, menu-active-hover-bg-color);
 }
 
 .search-reset {

public/css/themes/icinga-light-theme.less

@@ -5,8 +5,6 @@
 @gray-lighter: #EEEEEE;
 @gray-lightest: #F7F7F7;
 
-@menu-active-bg-color: #EDF7FC;
-
 @menu-2ndlvl-color: #7E8182;
 
 @low-sat-blue: #DEECF1;