Move CSS helper classes to root.css

static/styles/main.css

@@ -56,10 +56,6 @@ figure.bordered {
     border: 2px solid #eee;
 }
 
-.centered {
-    text-align: center;
-}
-
 h1,
 h2,
 h3,
@@ -331,12 +327,3 @@ pre {
         font-size: 7px;
     }
 }
-
-/*
- * Helpers
- */
-
-/* For <span>s that shouldn't break. */
-.nobreak {
-    white-space: nowrap;
-}

static/styles/root.css

@@ -281,9 +281,16 @@ nav.bulleted li::before {
 p { margin-block-end: 1em; }
 p:last-child { margin-block-end: 0; }
 
+/** HELPER CLASSES **/
+
+.centered { text-align: center; }
+.nobreak { white-space: nowrap; }
+
 .platter {
     background: var(--background-color);
     border: 1px solid var(--header-border-color);
     border-radius: 12px;
     box-shadow: 4px 5px 5px var(--header-box-shadow-color);
 }
+
+.visible { visibility: visible; }