ITCSS = Inverted Triangle CSS
This is a model developed by the consultant Harry Roberts. It's based on the idea that your CSS should be structured by defined metrics (specificity, reach, explicitness) rather than thematic groups.
I can see how this would work for a single site, or family of sites that share the same theme, but I can imagine it getting much more complicated when you have multiple sites or sections of a site with quite different themes sharing the same settings, tools, generic and element layers. Would be interesting to know if anyone has tried to combine this with Atomic design.