Custom CSS in Backlight
In CE4 and previous TTG products, custom css needed to be implemented through phplugins. That is a thing of the past.
In Backlight, custom css is enabled on a per template basis. Go to Designer > Templates > Page Templates. Click on Design next to your page template and scroll down to Advanced Customization.
Inside Advanced Customization you’ll see where to enable custom css and choose the file you want to use.
There are two demo custom css files included with Backlight. Feel free to enable these and see what they do.
But my suggestion is to create your own custom css file that contains your own customization. In a plain text editor, create a file and name it custom.css or whatever you want. Add any custom css to this file and upload it to your /backlight/custom/css/ folder.
Once you do, this new file will be available to use in any of your templates.