I don’t think there is a separate config key for the foreground color of the currently selected line.

The available theme options seem to include:

gui:
  theme:
    selectedLineBgColor:
      - blue
    inactiveViewSelectedLineBgColor:
      - bold

but I don’t see a corresponding selectedLineFgColor.

If the goal is better contrast, the supported workaround is to change the selected-line background style, for example:

gui:
  theme:
    selectedLineBgColor:
      - reverse

or use a specific background color:

gui:
  theme:
    selectedLineBgColor:
      - black

The docs mention reverse as useful for high contrast, so that is probably the closest built-in option for changing how the selected item looks without a dedicated foreground setting.

So the short answer seems to be: selected line background/style is configurable, but selected line foreground color is not currently exposed as a separate theme option.