[Foundation] Add highlight colour token for mark element

[millus]

The mark element in the Foundation elements layer (elements.css) has no styling yet, pending a new design token.

What's needed

1. Run Brand Midnight Sun through the colour generator to produce light and dark variant values suitable for a highlight background.
2. Add --eds-color-bg-highlight to Token Studio using those values.
3. Sync and build the token so it is available in variables.css.
4. Apply it in elements.css:

.eds-elements :where(mark) {
  background-color: var(--eds-color-bg-highlight);
}

Related

• PR feat(eds-core-react): create button label typography css #4660 — Button label typography CSS (where the TODO was introduced)

[millus]

This is part of the broader colour and token system work tracked in #4740.