fix search input visuality for Monokai

Author: amedora

browser/main/SideNav/SideNav.styl

@@ -184,7 +184,7 @@ body[data-theme="monokai"]
     background-color $ui-monokai-backgroundColor
     border-right 1px solid $ui-monokai-borderColor
   .search .search-input
-    background-color $ui-monokai-backgroundColor
+    background-color $ui-monokai-button-backgroundColor
     color $ui-monokai-text-color
     border-color $ui-monokai-borderColor