tobago-tree: custom elements; fix: margin-left for treeNodes in sheet

Issue: TOBAGO-1633
diff --git a/tobago-core/src/main/resources/scss/_tobago.scss b/tobago-core/src/main/resources/scss/_tobago.scss
index 38b92ca..ff14401 100644
--- a/tobago-core/src/main/resources/scss/_tobago.scss
+++ b/tobago-core/src/main/resources/scss/_tobago.scss
@@ -109,6 +109,14 @@
   }
 }
 
+@mixin treeNodeMarginLeftLevel() {
+  @for $i from 0 through 20 {
+    tobago-tree-node[data-tobago-level='#{$i}'] {
+      margin-left: #{$i}rem;
+    }
+  }
+}
+
 /* badge -------------------------------------------------------------- */
 .tobago-badge {
 }
@@ -1193,6 +1201,8 @@
 
 tobago-sheet {
   .tobago-sheet-cell {
+    @include treeNodeMarginLeftLevel();
+
     > tobago-tree-node {
       display: flex;
       align-items: center;
@@ -1497,20 +1507,12 @@
   margin-left: 7rem;
 }
 
-@mixin treeNodeMarginLeftLevel() {
-
-}
-
 tobago-tree {
+  @include treeNodeMarginLeftLevel();
+
   tobago-tree-node {
     display: block;
 
-    @for $i from 0 through 20 {
-      &[data-tobago-level='#{$i}'] {
-        margin-left: #{$i}rem;
-      }
-    }
-
     tobago-tree-select {
       display: inline;
     }