public TableCell.Builder addClass(final String cssClass) {
            if (!NullSafe.isBlankString(cssClass)) {
                if (cssClasses == null) {
                    cssClasses = new ArrayList<>();
                }
                cssClasses.add(cssClass);
            }
            return this;
        }
Add single class or space separated classes to the cell
stroom-core-client-widget/src/main/java/stroom/widget/util/client/TableCell.java