function _adjustFontSize(adjustment) {
        var fsStyle    = prefs.get("fontSize");
        var validFontSizeRegExp = new RegExp(validFontSizeRegExpStr);
        
        // Make sure that the font size is expressed in terms we can
        // handle (px or em). If not, simply bail.
        
         if (fsStyle.search(validFontSizeRegExp) === -1) {
            return false;
        }

        // Guaranteed to work by validation above.
        var fsUnits = fsStyle.substring(fsStyle.length - 2, fsStyle.length),
            delta   = fsUnits === "px" ? 1 : 0.1,
            fsOld   = parseFloat(fsStyle.substring(0, fsStyle.length - 2)),
            fsNew   = fsOld + (delta * adjustment),
            fsStr   = fsNew + fsUnits;

        // Don't let the font size get too small or too large. The minimum font size is 1px or 0.1em
        // and the maximum font size is 72px or 7.2em depending on the unit used
        if (fsNew < MIN_FONT_SIZE * delta || fsNew > MAX_FONT_SIZE * delta) {
            return false;
        }

        setFontSize(fsStr);
        return true;
    }
@private
Increases or decreases the editor's font size.
@param {number} adjustment  Negative number to make the font smaller; positive number to make it bigger
@return {boolean} true if adjustment occurred, false if it did not occur