Fix broken table headings

Bug report

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Checklist

• This issue is not a duplicate, I've searched the Issue Tracker
• This issue is not a question, I'm aware that I can submit questions via Mastodon, Twitter, Email, Reddit or Matrix
• This issue is a bug report, and not a content request, feature request, or anything other

What is the bug behavior?

Table headings are broken, no background color.

What are possible fixes?

Possibly linked to upgrade of Material for MkDocs (Issue #34 (closed)):

• _typeset.scss in new overrides/main.xyz.min.css has changed
• this defines table layouts
• fix by adding custom entry to layout.css

Do you have additional comments?

N/A