A RadioButton is a Checkbox

If you have this code and you are dealing with a RadioButton control
then the output CSS class will be HighlightedBackgroundCheckbox because
a RadioButton is a Checkbox.

Simple fix is to check its a radiobutton first.

namespace System.Web.UI.WebControls
public class RadioButton : CheckBox

foreach (string boundNameToCompare in boundNames)
{
    CheckBox cb =
        this.FindControl(this.TargetControlID) as CheckBox;
    if (cb != null)
    {
        cb.CssClass = "HighlightedBackgroundCheckbox";
        continue;
    }

    RadioButton rb =
        this.FindControl(this.TargetControlID) as RadioButton;
    if (rb != null)
    {
        rb.CssClass = "radio HighlightedBackground";
        continue;
    }

}