rewrite wrong variable name

searx/static/css/definitions.less

 /// Categories
 
 @color-categories-item-selected: @color-base;
-@font-color-categories-item-selected: #FFF;
+@color-categories-item-selected-font: #FFF;
 
 @color-categories-item-border-selected: @color-base-dark;
 @color-categories-item-border-unselected: #E8E7E6;

searx/static/css/style.less

 
 .checkbox_container input[type="checkbox"]:checked + label {
 	background: @color-categories-item-selected;
-	color: @font-color-categories-item-selected;
+	color: @color-categories-item-selected-font;
 }
 
 .search  {