Refresh an elements style in Javascript

Question

I have a table which the rows are loaded dynamically from a recordset. I also have used Javascript so that when a checkbox (not in the table) is checked it hides all the rows when column x = y, but I have a vertical border (really just a background image positioned to the right)which fades on the last row of the table using the CSS .sortable .dynamicrow:last-child td{

So when I check the box and the last one row is removed, the border doesn't fade properly basically I need to rerun/refresh the CSS.

In all I am asking how to refresh the style/CSS of an element in javascript.

   .sortable .dynamicrow:last-child td{
    background-image:url(../Images/tablevborderbottom.png);
    background-position:right;
    background-repeat:no-repeat;
}

Is the CSS I use to have the vertical border fade on the last row Sometimes the last row gets removed, so the second last row becomes the last row, but it keeps it's solid border and it doesn't fade out.

Another way of putting it is, after the page has loaded, because of Javascript sometimes the last-child gets removed, so child above it becomes the last-child, how do I apply the above CSS to the new last-child?