Add Nebula CSS toolkit

[rbrtsmith]

Added to list of UI toolkits.

I think it's relevant here because the toolkit evangelises best practices like ITCSS and BEM methodologies and allows for easy extension by enumerating on the variables (many of which are Sass Maps).

[dexteryy]

What we are doing in this document is not trying to catalogue the whole internet or every useful new project that looks good, also is not trying to compose dozens of "awesome lists" into a single one, but is trying to provide the complete view angle, reflect technological advance and help developers to acquire APIs, libraries, tools, services, best practices and learning resource which are not outdated and most commonly used.

The JS/web technology is like an ocean of stuff you don’t know. Simply collecting more projects will only exacerbate the Javascript Fatigue. So this document must stay lean and focus on the most frequent problems and the most commonly used stuff.

Which project belongs to "the most commonly used stuff" is not only decided by experience and observation, also by public data and web scraping scripts. In particular, the npm package statistics (like download count) are given more weight than Github repo statistics (like starring data) because npm statistics can better reflect the actual usage and popularity.

For example, you can use npmtrends.com to do some similar data analysis:

So based on the statistics of Nebula CSS, I must say it cannot be included at present.