Active CSS Code Editor
This is the code editor used on this site.
The github repo can be found here: https://github.com/Active-CSS/active-css-code-editor-component
Please note that the CSS in the code editor cannot itself be changed by using the code editor. This is because the CSS for the code editor is loaded by a file link, rather than by using inline content.
The main reason this is being displayed in the code editor is so that you can see the code involved in building a component like this.