Checkout: Order Summary / Coupon panel heading (disclosure triangle) background is white in Twenty Seventeen dark mode

[haszari]

Describe the bug

Twenty Seventeen has a dark colour scheme. When this is active, there's a white box around Order Summary and Coupon Code?:

To reproduce

1. Set up a page with Checkout block.
2. Install & activate Twenty Seventeen theme (observed issue with Version: 2.3).
3. Customizer > Colors > Color Scheme: Dark and publish changes.
4. On front end, add stuff to cart and view checkout.
5. See white boxes.

Expected behavior

No white boxes - these should look like regular headings on page background colour.

Context

• WordPress v5.4.2

This might need to be fixed in core/Twenty Seventeen, reporting here as a first step.

[senadir]

This should be fixed in #2981