This is a followup from bug 1574520.
Quoting from gl:
I am not quite sure we should fix the border colours with this patch. Using --theme-splitter-color here doesn't seem correct to me since we used to have an opacity of .1, and the border was never really visible in the light theme. My suggestion would be to move this change to a different bug and we can have Victoria's UX input.
Currently the border color is set to
rgba(255, 255, 255, .1) which is undistinguishable in white theme, and can only be seen in dark theme.
My suggestion is to change it to something like
-theme-splitter-color so it could be seen in white theme as well.
See attached for before & after the change for white and dark themes.