Unless you are absolutely sure that you will never ever need to modify any of the theme files you should install and activate a child theme for Lenox.
For your convenience Lenox comes with a ready to use child theme. You’ll find the “Lenox child” in Appearance->Theme if you have already installed it or install lenox-child-theme.zip file. Its files structure is as in the image below.
If you want to modify the theme css, you’ll have to edit the style.css file.