ScreenTextEntry

ScreenTextEntry is used when StepMania requires you to input some text. It is typically called from the source code.

Common
Starting in StepMania 4, you could theme ScreenTextEntry.

and  have the basic metrics (X, Y, OnCommand, OffCommand).

sm-ssc
As of sm-ssc v1.2.1, themers can use ScreenTextEntry in their themes via Lua.