The table cell, which contains the hierarchy expand/collapse button has a CSS class of rgExpandCol. You can use this CSS class to set a background color of your preference.
If the expand/collapse button disappears when you select a row, then there is something wrong with the custom skin. Please send it (including the CSS file and images) if you need further assistance.
Generally, the Visual Style Builder does not offer "design time" means to set every possible style in a skin. Sometimes you have to edit the CSS code itself. In such cases you need to know the control's CSS classes:
the Telerik team