fix(accordion): margin and padding of .item.accordion within power menu

- due to .ui.accordion .accordion rule from semantic ui we get different
  values for this particular item.
  By importing the default value if found we can preserve the original
  value of padding and only if not found provide a fallback padding

src/components/manage/Blocks/Accordion/editor.less

@@ -1,8 +1,13 @@
 @type: 'extra';
 @element: 'custom';
+@itemVerticalPadding:   0.875em;
+@itemHorizontalPadding: 1.25em;
 
 @import (multiple) '../../theme.config';
 
+// optionally import menu variables for itemVerticalPadding values
+@import (optional) '~volto-themes/default/collections/menu.variables';
+
 @borderColor: rgba(120, 192, 215, 0.75);
 
 // Edit
@@ -77,8 +82,10 @@
 
   .power-user-menu {
     .ui.menu {
-      .item {
-        padding: 0.875em 1.25em;
+      .accordion {
+        // override padding and margin from semantic .ui.accordion .accordion
+        padding: @itemVerticalPadding @itemHorizontalPadding;
+        margin: 0;
       }
 
       .title {