AV: A11y HCM issue with Guidance in psammead-media-player

[HarveyPeachey]

Is your feature request related to a problem? Please describe.
In the dev a11y swarm done for #2517, a few issues were found as documented in this comment:

One of these issues is with Windows HCM. Currently, with the implementation of guidance, the current opaque overlay is causing the placeholder image to not be visible which it should be in High Contrast Mode.

Describe the solution you'd like
Our implementation for the overlay is using background-color: rgba(34,34,34,0.75);. From a brief bit of investigation, it looks like background-color in Windows HCM is being replaced by a block colour hence the placeholder image not appearing.

We need to find a way to implement the opaque layer without blocking the placeholder image behind it in HCM.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Testing notes
[Tester to complete]

Dev insight: Will there be any potential regression? etc

• This feature is expected to need manual testing.

Additional context
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