Would be nice to have this:

<va-select css-variables="{ '--va-select-background-color': '#ff0ff0' }" />
//or
<va-select css-variables="{ 'select-background-color': '#ff0ff0' }" />

To make it possible we need to move CSS variables from :root to .va-component-root-class.

This can be turn to some component style service.