Remove vertical borders from Universal Theme Interactive Reports

In Template Options of Classic Report region you can set property to see only horizontal borders (property Report Border):

What about Interactive Reports? There's simple way to do it. Just open Theme Roller add this two lines of CSS in Custom CSS property:

  .a-IRR-header{
    box-shadow:none
  }
  .a-IRR-table td{
    border-left:1px solid transparent
  }

*Tested on APEX 5.0.1.00.06.