When are theme's dim terminal colors used?

[gim913]

When are theme's dim terminal colors (i.e. terminal.ansi.dim_red) used?

Inside theme, I have deliberately set ALL dim colors to FF00FF (fuchsia/magenta), I thought dim colors will be used when CSI 2; xx m is used, but it doesn't seem so.

CMD:

NU:

I'm guessing this is the part that handles it, and I see get_colors_at_index,
and the comment above even states, "values > 255 are for compatibility with alacritty:Colors interface".

So when/where exactly are they used?