fix(tree): move indigo tree background to the node (#2173)

Co-authored-by: Radoslav Karaivanov <rkaraivanov@infragistics.com>

Author: didimmova

src/components/tree/themes/shared/item.indigo.scss

@@ -5,6 +5,8 @@ $theme: $indigo;
 
 :host {
     --igc-tree-indentation-size: #{sizable(rem(24px))};
+
+    background: transparent;
 }
 
 [part~='indicator'] {
@@ -18,6 +20,7 @@ $theme: $indigo;
     border-radius: rem(4px);
     margin-block-end: pad-block(rem(4px));
     padding: 0 pad-inline(8px, 12px, 16px);
+    background: var-get($theme, 'background');
 
     &::after {
         border-radius: rem(4px);