The installer can't auto-detect the console colors on Mac (see examples one, two, three). Instead, let the player choose a theme in the installer and optionally preconfigure SMAPI to use the same theme.

Maybe add a line like this at the top, using the opposite color for the parenthetical text so it's visible when the current theme's text isn't.

SMAPI will choose the best colors automatically (press T to use light text instead).

Alternatively, add an extra installer question on Linux/Mac. The question and instructions should be unformatted, and the answers should be shown in the light/dark theme:

Which text looks more readable?
   [1] This text for light backgrounds.
   [2] This text for dark backgrounds.

Type 1 or 2 and press enter to continue.

Done in develop for the upcoming SMAPI 2.7. This revamps the installer to make multiple steps less confusing, and adds an extra question for players on Linux/Mac: