How to remove rich:dataTable header background

The rich:dataTable header has a gradient by default. To remove it, add the following in your css file:

.rich-table, .rich-table-header, .rich-table-header-continue,
  .rich-table-footer, .rich-subtable-footer {
     background-image:none;
     background-color:transparent;
}     

If you just trying to apply bg color you also should not redefine bg image with 'none';