Lightly fade buttons to add child to a non-structure variable

newIDE/app/src/VariablesList/VariableRow.js

@@ -94,7 +94,7 @@ const ThemableVariableRow = ({
   }
   columns.push(
     <TreeTableCell key="tools" style={styles.toolColumn}>
-      <IconButton onClick={onAddChild}>
+      <IconButton onClick={onAddChild} style={isStructure ? undefined : styles.fadedButton}>
         <AddCircle />
       </IconButton>
     </TreeTableCell>

newIDE/app/src/VariablesList/styles.js

@@ -24,4 +24,7 @@ export default {
   emptyExplanationMessage: {
     textAlign: 'left',
   },
+  fadedButton: {
+    opacity: 0.7,
+  },
 };