Moodle uses a lot of YUI technology for various things. The YUI project has started working on a modern CSS framework called "Pure". Moodle should add a theme to core using this framework with the aim of having something beta quality for 2.6.
The Pure framework does not (yet?, see https://github.com/yui/pure/issues/68) use a pre-processor so currently a major blocker is improving the renderers so that you can use Pure's HTML in standard components. With a pre-processor you can at least try to map the Pure CSS to current Moodle HTML with some limited success as demonstrated by the Bootstrapbase/Clean theme that does this. But for most areas the best and easiest approach is to improve the core renderers so that Moodle logic is separated from the HTML writing parts. (This would also massively benefit the Bootstrap-based themes or future projects based on any other 3rd part framework's CSS).
The above is the main reason why this should be done in core, rather than developed externally. It requires changes to core in order to be possible.
Other issues include it's use of the Font Awesome icon font, it's use of a grid so this this meta bug intends to track them so that any interested parties can work together on them.