Increase sketchbook tree indentation to reflect design system (arduino#1148)

Author: francescospissu

arduino-ide-extension/src/browser/widgets/sketchbook/sketchbook-tree-widget.tsx

@@ -27,7 +27,7 @@ import { Sketch } from '../../contributions/contribution';
 import { nls } from '@theia/core/lib/common';
 
 const customTreeProps: TreeProps = {
-  leftPadding: 20,
+  leftPadding: 26,
   expansionTogglePadding: 6,
 };