fix(themes): correctly set carbon--theme when nesting themes

[joshblack]

Noticed this with the work @janhassel is doing over in: #5382

It seems like we're not correctly setting $carbon--theme when nested inside of the carbon--theme mixin. This meant that the following was working incorrectly:

$carbon--theme: ( value-01: #ffffff );
$carbon--theme--dark: (value-01: #000000 );

@include carbon--theme($carbon--theme--dark) {
  @debug map-get($carbon--theme, 'value-01'); // Incorrectly logs #ffffff
}

This PR updates the builder for our theme mixin to correctly handle this case by setting the current $carbon--theme value at the beginning of the function to a local $parent-carbon-theme variable and is then reset to that value at the end of the function.

Changelog

New

Changed

• Update theme builder to correctly set $carbon--theme when nested in the carbon--theme mixin

Removed

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[emyarod]

looks good to me

[joshblack]

@dakahn let me know if you want to go over these changes!