CSS has Custom Properties which can be used both in a CSS file as well as an inline style.
A custom property has the following signature: --*.

This does not appear to be supported by Html.Attributes.style?

I'm using the following code,

applyTheme theme =
    style
        [ ( "--theme-primary-color", theme.primaryColor )
        , ( "--theme-secondary-color", theme.secondaryColor )
        ]

applied like div [ applyTheme theme ] [].

This produces no style attribute on the element. Other CSS properties do.

Would it be possible to add support for this?

https://www.w3.org/TR/css-variables/