Some color variables are defined outside of _colors.scss

[actionless]

Here:

materia-theme/src/gtk-3.0/3.22/sass/_common.scss

Lines 1578 to 1581 in aa9901e

	$opaque_fg_color: if($variant == 'light', $grey_900, $white);
	$opaque_secondary_fg_color: if($variant == 'light', $grey_600, mix($white, $base_color, percentage(0.7)));
	$opaque_disabled_fg_color: if($variant == 'light', $grey_500, mix($white, $base_color, percentage(0.5)));
	$opaque_divider_color: if($variant == 'light', $grey_300, mix($white, $base_color, percentage(0.12)));

That's breaking recoloring the theme's highlighted dropdown.

[nana-4]

You are right. I'll move those (and other remaining) color variables to _colors.scss.
Thanks for your report!