The easiest way to start is not from scratch, but by forking (the built-in fallback theme). Copy the Default folder within your Themes directory, rename it, then modify Graphics\ScreenTitleMenu background.png . As you learn Lua, you can replace individual screens.
Making a theme for StepMania 5.1 is easier than it looks:
For many players, StepMania is a tool to recreate the arcade experience at home. Themes like and DDR A3 by Curilang aim for near-perfect 1:1 recreations of modern Dance Dance Revolution cabinets. These projects often involve years of development to emulate complex scoring systems and menu transitions that older themes could not achieve. Compatibility and Continuity Give Your StepMania a New Style
: Aimed at streamlining game flow by removing clutter and exaggerated animations. Key Features