That totally depends on what you mean by design code.
To tidy your code up a bit, I would use the Anchors and designer script as much as possible when feasible to do so, try not to repeat the same code multiple times, share your code through the code modules and classes, variables that are the same in multiple modules can be store on your Starter Module (this includes bitmaps(images), maps, strings etc), don't load the same image multiple times, keep subs and loops as short as possible, there's way to much to mention.
You say that you basically have one screen per activity which is correct, just try to keep everything nice, tidy and not repetitive.
Watch Erels videos called 'Code Modules and Classes' and also watch the 'Code Flow' video.
https://www.b4x.com/etp.html
Learn and enjoy...