Create hover color variable

Signed-off-by: Marco Ambrosini <marcoambrosini@pm.me>

core/css/variables.scss

@@ -35,7 +35,8 @@ $color-main-text: #222 !default; // Not #000 for better readability
 $color-main-background: #fff !default;
 $color-main-background-translucent: rgba($color-main-background, .97) !default;
 
-// used for different active/disabled states
+// used for different active/hover/focus/disabled states
+$color-background-hover: nc-darken($color-main-background, 4%) !default;
 $color-background-dark: nc-darken($color-main-background, 7%) !default;
 $color-background-darker: nc-darken($color-main-background, 14%) !default;