[Range] Allow styling individual thumbs using `:nth-child()`

[TheOneTheOnlyJJ]

This is a very simple feature request.

The current explainer says:

These pseudo-elements can be used in combination with other CSS selectors, such as :nth-child(), to provide granular control over individual ticks, labels, and segments.

I believe ::range-thumb should also allow this granular control, using the same approach.

[lukewarlow]

Something interesting here is that currently you can't use structural pseudo classes on pseudo elements. So :nth-child() wouldn't work on any pseudo elements. I'm not sure if this is a restriction we can necessarily loosen. Though perhaps with base appearance having a defined structure it's plausible.

[brechtDR]

In current form, I had it like this in the explainer based on that same statement

[TheOneTheOnlyJJ]

While the exact syntax for this is still not finalized and may change, I have opened this issue to request the individual customizability of range thumbs.

It is not about the exact syntax, but about adding the thumbs alongside the other individual, customizable range sub-components.