Setting $body-bg to a CSS variable in Bootstrap 5

[fefrei]

In a Bootstrap 4 app, I'm using Bootstrap like this:

$body-bg: var(--body-bg);
$body-color: var(--body-color);
// more variables set like this

@import "../node_modules/bootstrap/scss/bootstrap";

This makes implementing a dark mode dead simple: My JS sets an attribute on the body according to the requested theme (based on in-app user settings and the OS theme), and some simple CSS applies proper colors:

body {
    --body-bg: #FFF;
    --body-color: #111;
    // and so on
}

@media screen {
    body[data-theme="dark"] {
        --body-bg: #000;
        --body-color: #EEE;
        // and so on
    }
}

This works great. Not all SCSS variables are customizable like this (notably, my popovers are always black-on-white) because some go into SCSS expressions like darken(), but most are – and I was expecting this to be the direction Bootstrap moves in as a whole, base on discussion in many issues like #26596.

Now, I'm struggling to upgrade to Bootstrap 5, because now, setting $body-bg: var(--body-bg); is no longer possible, as the body background color makes its way into _table-variants.scss where it's used in an opaque expression:

SassError: argument `$color-2` of `mix($color-1, $color-2, $weight: 50%)` must be a color
        on line 150 of node_modules/bootstrap/scss/_functions.scss, in function `mix`
        from line 150 of node_modules/bootstrap/scss/_functions.scss, in function `opaque`
        from line 4 of node_modules/bootstrap/scss/mixins/_table-variants.scss, in mixin `table-variant`
        from line 134 of node_modules/bootstrap/scss/_tables.scss
        from line 22 of node_modules/bootstrap/scss/bootstrap.scss
        from line 33 of src/bootstrap.scss

This is pretty bad – changing the $body-bg is one of the more important aspects of having a dark mode 😉
Is there a good way to circumvent this? (Or would that even be considered to be a Bootstrap bug – it's pretty reasonable to call this a regression, I think?) Is there another approach to implement CCS-based dynamic theming?