I think most people prefer to use the native Decker application on a computer to author their projects rather than any kind of web Decker, if the option exists.
But it is possible to reduce the amount of extra space around the program on a phone screen.
If you hold your phone in a horizontal rotation and select Decker > Fullscreen in the menu bar at the top it brings everything in closer.
And optionally you could also select Decker > Toolbars, which makes a variety of drawing tools available in the remaining gaps on the sides.
(Even if you're not drawing anything it can be useful to be able to switch between Interact mode and Widget mode with a single click -- or tap -- with the buttons at the top of the left side toolbar.)