Would be incredible to be able to define color variables through the use of widely-supported color-mix() etc. in custom css and have them be visible in the designer, not just the published site.

For context core colors could be defined in Webflow's native variables such as 'success', then used for color-mix() in custom css like so:

--success-surface: color-mix(in oklab, var(--swatch--success) 15%, white);

https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/color-mix