Custom table header

Here is a script that you can run on a selected table; you only need your cursor anywhere in the table. It will prompt you for one of the document’s Ruling Formats. When you select one, it will apply it to the table row’s column separators.

main ();

function main () {

    var doc, tbl;

    // Test for an active document.
    doc = app.ActiveDoc;
    if (doc.ObjectValid () === 1) {
        // Make sure the cursor is in the table,
        // or one or more cells are selected.
        tbl = doc.SelectedTbl;
        if (tbl.ObjectValid () === 1) {
            processDoc (doc, tbl);
        }
    }
}

function processDoc (doc, tbl) {

    var rulingFmts, index;

    // Get the document's ruling formats.
    rulingFmts = getRulingFmts (doc);

    // Prompt the user with the document's ruling formats.
	index = ScrollBox ("Please choose the Ruling Format:", 
	    getStringsFromArray (rulingFmts.names));
	if (index !== null) {
	    // Apply the selected ruling format to the heading columns.
	    processTbl (rulingFmts.objects[index], tbl, doc);
	}
}

function processTbl (rulingFmt, tbl, doc) {

    var row, cell;

	if (app.Displaying === 1) {
        app.Displaying = 0;
    }

    // Loop through the table's heading rows.
    row = tbl.FirstRowInTbl;
    while (row.RowType === Constants.FV_ROW_HEADING) {
        // Process each cell.
        cell = row.FirstCellInRow;
        while ((cell.ObjectValid () === 1) && 
            (cell.NextCellInRow.ObjectValid () === 1)) {
            applyColumnRuling (cell, rulingFmt);
            cell = cell.NextCellInRow;            
        }
        row = row.NextRowInTbl;
    }

    if (app.Displaying === 0) {
        app.Displaying = 1;
        doc.Redisplay ();
    }    
}

function applyColumnRuling (cell, rulingFmt) {

    cell.CellOverrideRightRuling = rulingFmt;
    cell.CellUseOverrideRRuling = 1;
}

function getRulingFmts (doc) {

    var rulingFmts, fmt;

    rulingFmts = {objects: [], names: []};

    fmt = doc.FirstRulingFmtInDoc;
    while (fmt.ObjectValid () === 1) {
        rulingFmts.objects.push (fmt);
        rulingFmts.names.push (fmt.Name);
        fmt = fmt.NextRulingFmtInDoc;
    }

    return rulingFmts;
}

function getStringsFromArray (array) {

	// Convert an array into a list of Strings.

	var count, i, strings;

	strings = new Strings ();
	count = array.length;
	for (i = 0; i < count; i += 1) {
		strings.push (array[i]);
	}

	return strings;
}

Unfortunately, you can't accomplish this with a table style natively in FrameMaker. You will need to create a ruling style set to white, and apply it via Custom Ruling and Shading.

If you have a lot of tables, this is a lot of work but ​@frameexpert can likely help you automate it with a script. 

Can you post a screenshot? Thank you.