I have already noted here and here that Prime have removed themeroller support in favour of a paid for theme designer tool, and more significantly, have removed there actual new theme SCSS api from open source, so this is paid for as well, preventing you from using their new API with manually designed themes.
My solution is to pick one of the free themes which is nearest to the look I want, and then rebadge its compiled css (which you do get a copy of) as SCSS, and then customise that.
Obviously the raw CSS has a lot of duplication compared to the original SCSS. My original strategy was to copy raw rules to the end of the file in a new section, and modify them as required to override the original rules, by making more specific as required (e.g. by adding a p.component class). This would then allow them to be matched to their originals, which sounded attractive from a correlation/derivation point of view.
In the end I gave up on this idea. The section with the modified rules was getting messy with all the duplication, and was harder to make sense of. I therefore adopted a new strategy which was to copy all the rules and modify as required, and then do some fairly straightforward merging of them into single SCSS rules. This dramatically simplified the modifications and made them clearer to read, so I have stuck with this idea going forwards.
There have been oddball cases where I have taken a different approach, and actually edited the original generated css directly. For example, there is a media query to switch the menu bar into hamburger only mode at a certain fixed viewport width. I did not want this, so as the media query covered a lengthy section of css, I removed the media query directly, replacing it with a class based rule so I could still switch in hamburger mode dynamically when I wanted. This still encapsulated the same original css, and seemed the tidiest approach in this case.
