Compute icon size based on context

This insure when .smaller is applied, it stays visually nice

Author: cderv

src/resources/formats/revealjs/quarto.scss

@@ -953,9 +953,9 @@ kbd {
     }
 
     .callout-icon::before {
-      height: 2rem;
-      width: 2rem;
-      background-size: 2rem 2rem;
+      height: 1.25em;
+      width: 1.25em;
+      background-size: 1.25em 1.25em;
     }
   }
 
@@ -1070,8 +1070,8 @@ kbd {
       }
     }
     .callout-icon::before {
-      margin-top: 0.5rem;
-      padding-right: 0.5rem;
+      margin-top: 0.25em;
+      padding-right: 0.25em;
     }
   }