osx - Can not modify header cell of Table View -

my cocoa app has view-based table view.

the table displays week of calendar, each column day (say) sunday saturday.

i highlight column corresponds "today" in way; ideally, give colored background textfield, rounded corners; this:

(this trivial achieve in ios. on macos, cells etc. instead of views , layers, seems more complicated...)

however, not able change header cell's text or background color. code below (swift 3) is executed, column headers displayed default colors :

let columns = tableview.tablecolumns  // (currentweek array of nsdate) (index, day) in currentweek.enumerated() {              let column = columns[sundayindex + index]      if shared.calendar.isdateintoday(day) {         // these 2 lines executed, but...          column.headercell.textcolor = nscolor.red          // ...no effect          column.headercell.backgroundcolor = nscolor.black          // ...no effect     }      // works every row:     column.headercell.stringvalue = formatter.string(from: day)   }