How to modify a Sass variable, based on another pre-existing one?

[thany]

From the guide:

$container-padding-x is now 16px instead of 12px as in v2. You can replace it with $spacer * 3 to get to the previous look.

How do I go about that?

The problem is dat when I try this, the compiler says $spacer doesn't exist. And it rightfully doesn't, because it's not imported. So where do I import it from?

Currently I've got this:

@use 'vuetify/settings' with (
  $container-padding-x: 12px,
);

I've also tried with settings.$spacer and tried adding @use 'vuetify/settings'; but I've found no combination of trial&error that makes it work with $spacer.

I'd appreciate some help with this specific problem, but also an answer to the question in more general terms, if possible. And when an answer is available, I feel it's pretty important to update the documentation about Sass variables as well, because I can't make heads or tails of it.

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On a sidenote: $container-padding-x is a misleading as it actually sets all paddings, not just horizontal ones.
And another sidenote: $container-padding-x does not show up in the Variable API - why is that?