You're looking for StepMania 5.1 themes. Here are some popular ones:
Starlight (Project XX): Known for its high visual flair and modern arcade aesthetic, though it can be more resource-intensive and may have minor bugs in certain modes like Courses. stepmania 5.1 themes
The download was 4 gigabytes. For a theme. Leo clicked it. You're looking for StepMania 5
Lambda: This was developed as a modern, concise default theme for StepMania 5.1, offering a flat design and improved menu navigation. concise default theme for StepMania 5.1
The official StepMania forum and the #theme-dev channel on the StepMania Discord are invaluable resources.
can be found on community archives and GitHub repositories. Popular choices include: Simply Love (SM5)