Undefined Variable fix

[gladishd]

@import '@carbon/themes/scss/themes';
@import 'carbon-components/scss/globals/scss/styles.scss';

// Use the gray 100 theme
$theme: $theme--g100;
@include theme();

carbon-design-system/carbon-tutorial#3940

[tay1orjones]

Thanks for the heads up @gladishd, we're working to update the @carbon/theme docs in #11293