XF 2.3 Enabling style variant for our one style breaks the appearance - help needed, please

[Stuart Wright]

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Upgraded from XF 2.2.16 to 2.3.6 this morning (got up at 4am to do it).
Some elements of the site use a dark background as you can see. The header, the footer, category headers.
But when I enable style variants, I get the appearance here:

How do I implement style variants to give us dark mode and keep some elements of light mode as they currently are?
Thanks