Hello.
Is it possible to modify the templates with a code editor rather than using the ACP?
You can always copy and paste the content into your text editor, but other than that no.
You can also export theme as .xml -> edit it in text editor -> import it. But I don't think that's efficient at all and don't know why would that help.
I have often thought it would be very convenient if the ability were added to import/export HTML files as individual templates.