Sm-ssc

sm-ssc (also known as StepMania SSC) was a fork of the StepMania codebase as of StepMania 4 alpha 5. It became the base for StepMania 5. It is made by themers for themers, though it also contains some serious gameplay changes as well.

Versions
The last stable version of sm-ssc is v1.2.5. It should no longer be used, as the codebase was taken verbatim for StepMania 5.

There is a sm-ssc v2.0.0 in development, but only as a personal build for the members of the spinal shark collective. Regardless, some changes made in this fork are backported to mainline StepMania 5.

Theming for sm-ssc
sm-ssc is compatible with StepMania 4 alpha themes (usually via adding some lines to the metrics). It should be noted that sm-ssc themes are not directly backwards compatible with StepMania 4 alpha versions. In addition to new Lua bindings/commands, there are a few changes in screen order/naming.

sm-ssc themes and StepMania 4 (beta 1 and up) themes are NOT compatible.

ScreenInit
ScreenInit is the replacement for ScreenCompany. This affects metrics in  as well.

ScreenServiceOptions
Whereas StepMania 4 alpha 5's ScreenServiceOptions derives from ScreenOptionsSimple, sm-ssc's ScreenServiceOptions derives from ScreenOptionsSimpleService. When backporting a sm-ssc theme to SM4a5, be careful of the default theme's metrics, as this screen will almost always look wrong the first time you try it.

In addition to the menu looking odd, if you've copied the service menu entries from sm-ssc, expect SM4a5 to complain about missing strings and crash on certain options that don't exist in SM4 (System Direction, UI Options, etc.)

ScreenStageInformation
Instead of ScreenStage in StepMania 4 alpha 5, sm-ssc uses ScreenStageInformation. Other than that, they're pretty much the same. When backporting a sm-ssc theme to SM4a5, it's better to use ScreenStageInformation, as you won't have to redir/edit the SM4a5 default theme's ScreenStage BGAnimations.