adding z-index mixin and implimenting

ui/src/components/tab/style.css

@@ -1,3 +1,5 @@
+@import '../../util/mixins.css';
+
 .tab {
   display: inline;
 
@@ -20,7 +22,7 @@
     opacity: 0;
     position: fixed;
     width: 1px;
-    z-index: -1;
+    @add-mixin move-z -1;
   }
 
   & .panel {
@@ -49,7 +51,7 @@
     background: white;
     border-bottom-width: 0;
     padding-bottom: remCalc(11);
-    z-index: 1;
+    @add-mixin move-z 1;
   }
 
   & .input[type="radio"]:checked ~ .panel {

ui/src/util/mixins.css

@@ -16,3 +16,8 @@
     @mixin-content;
   }
 }
+
+@define-mixin move-z $amount:0 {
+  position: relative;
+  z-index: $amount;
+}