This patch adds SeparatorColor to DecorationRole. SeparatorColor is for separators and frames. The default color is the same as the color that the Breeze widget style currently produces with the Breeze colorscheme (75% Window NormalBackground, 25% Window NormalText).
It needs patches in the colorscheme editor and Breeze widget style to be useful. I will post those later.
Users sometimes ask for the ability to customize the color of separators.
When talking about how the next version of Breeze Dark should look, some developers prefer dark separators and some prefer light separators.
Since opinions about separator color are pretty divided, it seems like a good idea to make it customizable.