typst css: div color, background-color; translation of named brand colors

[gordonwoodhull]

This allows

:::{style="background-color: var(--brand-dark-grey); color: var(--brand-blue)"}
This div is blue on dark grey.
:::

producing

#block(fill: brand-color.dark-grey)[
#set text(fill: brand-color.blue); This div is blue on dark grey.

]

or produces a warning if the named brand color does not exist.

Parsing variable references isn't worse than parsing rgb, right?

cc @cwickham

[gordonwoodhull]

Odd, this test

tests/docs/smoke-all/2024/10/28/brand-typography-family-string

from Monday a97fcc0 was just failing today, get_typography in brand.lua needs to upgrade family string to typography object.

I don't understand how this change opens up that code path, but apparently it does.