Not sure how well this plugin is supported any longer. An alternative is the Code Snippets plugin.
The plugin Code Snippets which is very popular one can add PHP into. Is a good alternative to the CodeKit – Custom Codes Editor.
I decided to use this plugin because I would be able to easily add CSS and PHP code snippets to my web site. I am impressed with what I saw.
It starts out with a black background, but in the bottom left area I changed it to white. One can make various modifications to the editor environment. On the right side are options to where the code will affect. Some of these options are only activated through the PRO plugin.
At the top left of the editor is a drop down to where one selects the type of code one wants to add.