FYI, I figured it outā¦at least, one way to do it.
Seems itās not something that cybertron even sets, so itās using some default.
To address, I added to the .theme-dark selector at top:
--text-selection: #164f92;
Not sure if that particular color choice fits the vibe you are going for in cybertron (no doubt it can be improved to fit better), but it fixes my ability to see the selection well enough without being too garish, at least for now.