GtkCellRenderer ignores cell-background when selected

I can set the background colour of a cell in a GtkTreeView using the cell-background-rgba property, but this is ignored when the row is selected, even when the row selection colour has an alpha of 0.5. The result is shown here. In this case the selection colour is defined in the CSS as alpha(#cce8ff, 0.5).

What ought to happen is that the background colour should be blended with the transparent selection colour.

(In my application the user can choose the colour in question, so its not just a matter of specifying some custom CSS).