HighContrast theme: little highlight when the mouse hovers a column in trees

When user click on a the column header in Thunderbird or in Caja he will reorder the list. For low-vision people using the HighContrast theme it is possible to move the mouse a little too high and click unexpectedly on the column header.

The current behavior is when the mouse is over a column header, almost nothing happens. We'd like to have a visual feedback to indicate to the user that he is over the column header.