Custom shading using ExtendScript based on element attribute

I am rather new to ExtendScript, and working on a structured DITA FM document. In our tables, we use a background colour to highlight a specific type of information. Of course, this shading is contained only in the FM data, and is lost on roundtripping. For the <entry> elements of the cells that we want to be shaded, we have set the @outputclass=bgcolor.

Now, I am trying to use ExtendScript to locate the elements with the appropriate attribute, and then apply the custom shading. I can't figure out how to get the shading applied once I find the element. I am sure I am missing something really basic, but am completely stuck.

This is what I have so far...

// Remove existing shading modifications from cells…

var tbl = app.ActiveDoc.FirstTblInDoc

while (tbl.ObjectValid() ) {

  var row, cell

  row = tbl.FirstRowInTbl

  while (row.ObjectValid() ) {

  cell = row.FirstCellInRow

  while (cell.ObjectValid() ) {

  //changing the cell definition so that color properties are taken from table format }

  cell.CellUseOverrideFill = false

  cell.CellUseOverrideShading = false

  cell = cell.NextCellInRow

  }

  row = row.NextRowInTbl

  }             

  tbl = tbl.NextTblInDoc

};

// Get the document’s root element…

var elem = app.ActiveDoc.MainFlowInDoc.HighestLevelElement;

// Launch a function that walks the tree and shades the cell when appropriate…

walkTree(0, elem);

app.ActiveDoc.Redisplay (); 

// A recursive function that walks the tree…

function walkTree(level, elem){

    // Find elements where @outputclass has a value

     var allAtts = elem.Attributes;

     var AttVal = "";

     var i = 0;

     var attFound = false;

     while( !attFound && i < allAtts.length ) {

          if( allAtts.name == "outputclass" ) {

               attFound = true;

               if( allAtts.values.length > 0 ) {

                    AttVal = allAtts.values[0];

               }

          }

          i++;

     }

    // For @outputclass=bgcolor, shade the cell

    if AttVal == "bgcolor") {   // THESE ARE THE FOUR LINES THAT DON'T WORK

        this.cell.CellOverrideShading == app.ActiveDoc.GetNamedColor("Background"); 

        this.cell.CellUseOverrideShading = true; 

        this.cell.CellOverrideFill = 100;

        this.cell.CellUseOverrideFill = true; 

  }    

  level = level++;

  var child = elem.FirstChildElement;

  while (child.ObjectValid()) {

  walkTree(level, child);

  child = child.NextSiblingElement;

  }

};

The script removes existing shading properly, but does not apply the new one.  Does anyone have any suggestions?

...C