The second section found in Developer Mode allows you to define the CSS for your theme. To keep your theme flexible, properties found in your Theme CSS should reference your LESS variables whenever possible. Incorporating LESS variables in your theme css effectively ties your CSS into the Customize section of the Appearance Panel. This is great for the average person who will be using your theme and will likely want to make tweaks to colors, images, font, etc. When themes don’t incorporate LESS variables, changes to the theme need to made via additional CSS which doesn’t make for the best user experience.