Css is a basic element that can allows you to embed raw CSS code into the simulation.
You can find the "Css" element in the elements pane, and add it into your page. It looks like:
Different than other elements, Css element is not visible in HTML5 simulation and exported image/PDF, instead it inserts its content (CSS code) into the HTML5 simulation. Here is an example of Css content:
When you export your plot into HTML5 simulation, the CSS block will be inserted into the simulation as well.
Remarks: Embedding inappropriate CSS code into your plot may get unexpected result in the HTML5 simulation. It is better not to do so, unless you have to, and you really know what you are doing.