Internal Code snippet variable is causing issues with custom themes

[tw15egan]

Based on convo in Slack https://ibm-studios.slack.com/archives/C046Y0YUD/p1601663658098800

For some reason, when a user tried to set a custom $ui-01 using css custom properties, the value was not updated. We use an internal token in the code snippet mixin, which is set to $ui-01 by default. The inline code snippet updated properly but does not use the internal mixin.

Wondering if we should just update the mixin the use $field-01 directly (the default), and not $snippet-background-color;, which is not used anywhere else.

carbon/packages/components/src/components/code-snippet/_mixins.scss

Line 19 in ad4d9a2

background: $snippet-background-color;