使用javascript在css中生成更亮/更暗的颜色

let say I have #404040 color code. How to generate a new color code which is either lighter or darker by 20% (or given x%)? I need this for generate a hover color in a dynamic site (which color is changing using theme). Therefore it is not possible to use another class or :hover with predefined class.let say I have #404040 color code. How to gener