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 3.5.15.30\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.

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