Unfortunately, there is no ability to ship styles with phrases, so this would be a custom edit.
A simple approach would be to go to Appearance -> Phrases -> Add phrase
"design_by"
Depending on how your languages are setup, you may need to add this into each language. If you're using a child language, creating it at the top level will trickle down.
Then open PAGE_CONTAINER, click into the editor and hit ctrl-f, search for: Design by
Replace that with:
[code[{{ phrase('design_by') }}[/code]
Sorry, I do wish there was an easier way. Maybe 2.3 will bring in more functionality for styles.