Make softkey height into a variable

Not that we'll need to change the height, but we need to refer to
it in some places and variables makes it more clear what exactly
we're referring to (as opposed to bare numbers).

public/kaios-native-ui.css

@@ -20,6 +20,7 @@
 
   --softkeys-background: #cccccc;
   --softkeys-color: #323232;
+  --softkeys-height: 3rem;
 
   --item-background: #ffffff;
   --item-text-color: #323232;
@@ -651,7 +652,7 @@ body {
   /* grid-gap is an old way of saying column-gap */
   grid-gap: 0.5rem;
   display: grid;
-  height: 3rem;
+  height: var(--softkeys-height);
   background: var(--softkeys-background);
   color: var(--softkeys-color);
 }