Up to SP13:
Open the menu Tools → Options and set the desired color for the text editor.
As of SP14:
Open C:\Program Files\CODESYS 184.108.40.206\CODESYS\Themes
The color scheme is available as a JSON file in the "themes" folder.
You could edit or copy it and paste it with a new name in the folder mentioned.
They are then read in again and you could also exchange them.