After some searching, it seems it’s impossible to have have a gutter colour without any highlighting, but to still have a highlighted line in the main text itself.
There is also no combination of colour schemes that can simulate this. At best, the gutter colour can be set to the same as the line highlight colour—but then you end up with what looks like a wide vertical bar down the left side of the window, with a perpendicular bar, highlighting the active line of text, that bisects it.
Therefore, what I would like to see is independent preferences:
“highlight_line”: true,
“gutter_highlight_line”: false
It would also make sense to have independent theme settings for specific colour styling:
lineHighlight
#FFFFFF
gutterLineHighlight
#000000