Yes, this is the default behavior.
There is the highlight color, which is the value you set in the color scheme,
In your case it would look like:
$highlight: #1a1a1a;
And the usage would be like:
background-color: $highlight;
And then there are three color transformations based on the $highlight and $secondary values and they are:
$highlight-low: dark-light-diff($highlight, $secondary, 70%, -80%);
$highlight-medium: dark-light-diff($highlight, $secondary, 50%, -55%);
$highlight-high: dark-light-diff($highlight, $secondary, -50%, -10%);
And these are used like so:
.overridden {
background-color: $highlight-medium;
}