The master branch, as of April 10, 2017, is now the 5.0 branch that was created by Riakodoadm, and the previous version has been moved to a new "5.2" branch. This mirrors the similar rollback that StepMania itself performed.
Since the new 5.1 is meant to be more compatible with 5.0.x, you no longer need a nightly build to run this theme. However, the edit mode menu is broken because it will require some backend support code to be backported from the 5.2 branch of StepMania.
Lambda (formerly FiftyOne) is a project to create a new, prospective default theme for StepMania 5.1.
The current default theme dates back to the original sm-ssc fork, and barring design tweaks over the years, has remained relatively unchanged. As 5.1 has some of the most significant changes to StepMania in quite a long time, it is only appropriate that a new default theme be constructed to showcase its advancements.
Among the technical goals for Lambda are to provide the same level of functionality as the current default theme, as well as familiarities, whilst futureproofing it for future changes by reducing its reliance on legacy components. Lambda is also meant to reflect modern design trends, utilizing a clean and gradient-based aesthetic with a friendlier and more inviting look. The theme is also meant to have a more vivid and less duochromatic appearance than the previous default. To provide more efficient use of space on larger displays, Lambda uses a base resolution of 720p as opposed to 480p. Yes, this may break some stuff, but it's not like SM 5.1 has already broken stuff for the sake of progress to begin with.
I'd say it's about 55% done. Needs a bit more eye-candy, some screens and modes still need to be fixed up/polished, and Edit mode needs to be worked on (especially after 5.1 is modified to support the new edit menu scripting).
Feel free to contribute and make pull requests.
Inkscape SVG source code is available for the majority of graphics created in the _src folder. Some of the original mockups are also included for reference, although they are meant primarily for reference purposes due to changes in the theme's style since these mockups (such as the removal of rounded corners and increased use of chamfered edges and diamond motifs).
The main fonts are the current version of Roboto and Noto Sans https://www.google.com/design/spec/resources/roboto-noto-fonts.html, and Open Sans (although, preferably, Noto Sans should eventually be used in any location Open Sans is currently used). Certain accents make use of the font Overpass. http://overpassfont.org/
As it incorporates existing code from StepMania, and is intended to possibly be part of the main distribution, Lambda is licensed under the terms of the MIT License.