Where is the "--bricks-text-dark" colour variable defined?

[Sridhar]

It is present in these files in Bricks theme:

To override it on the front end, you could add this sample CSS in your child theme’s style.css or at Bricks → Settings → Custom code → Custom CSS or in the Customizer:

:root {
	--bricks-text-dark: #111;
}

To override it in the Bricks editor, add it at Bricks → Settings → Builder → Builder mode: custom. Builder Mode (Custom) – Bricks Academy