Though it doesn't work if the background is white and foreground is black.

What would be the proper fix?

There is no concept of a brighter foreground though as foreground is not necessarily black or white. Not sure if it could be fixed.

There is no concept of a brighter foreground though as foreground is not necessarily black or white. Not sure if it could be fixed.

nothing "can't be fixed" when they are wrong, you just have to do the right thing

if other terminals do it properly

there is no reason Terminus can't do it too, it's supposed to do what a terminal do

Sublime Text doesn't have a proper way to allow us to dynamically bold a region, so we could only support brighten bold text. However, unlike the canonical ANSI colors, there is no brighter color of the default foreground. It is actually correct for not "bolding" the default color.

I think the only wrong thing is that Terminus doesn't support bold, but as I have mentioned above, it is due to Sublime Text limitation. If we are willing to assume that the foreground is always the ANSI color number 8 (i.e, white), we could use ANSI color number 16 (i.e. bright white) to "bold" the foreground. That assumption doesn't always hold, though.

Could the choice of which colour to use for such text be delegated to the Terminus themes? Add an additional property to the JSON and let the theme designer (or the user configuring their "user" Terminus theme) decide which of the colours to map it onto.

I applied @frou's suggestion
I made it so it grabs the default color via settings default_color, and default to white