As described in bug #396091, the Breeze-dark theme is often neglected in updates. This patch fixes #396091 and some additional, related inconsistencies, enables sharing the basic stuff like shape, size, style, etc. of components in a common single css file while keeping the colour definitions separated from that in different files for both the light and dark theme.
This allows easier changes, easier maintenance because of less LOC, less duplicate code as well as easier extensibility for potential additional colour schemes like light or high-contrast.
I only did this for GTK 3.20 for now. If you like the effort, it could (and maybe should) get extended to other versions.
Further potential steps in the same direction of saving code would be going back to SASS, which is used by many other popular GTK themes and was used by Breeze-gtk as well in the past. The only downside of that is that the SASS source files would have to be "compiled" to CSS prior to packaging.