Before you start changing your current theme settings, you have to perform a few things.
Customizing the font and color of the editor is only available in the New ABAP Editor. If you don't use the New ABAP Editor, you have to enable it first. Use the menu ABAP Editor -> Utilities -> Settings -> Editor tab. Here let's choose the Front-End Editor (New) option. If you are ready, apply the settings.
After that navigate to this folder: C:\Users\[YOUR USER]\AppData\Roaming\SAP\SAP GUI\ABAP Editor\
Here I recommend you to create a backup of your original settings. The font and color settings are stored in the file: abap_spec.xml. Let's create a copy of it, and save it somewhere else.
Later if you want to restore your default settings, just replace the actual abap_spec.xml file in the mentioned folder with your backup version.