Just like we did with SCSS variables, we use the `--main-header` CSS
variable and, if it isn't defined, we use the `--brand` CSS variable
instead.
Note that we're still using the `inverted-selection` mixin based on the
default `$main-header` color, so it's possible that we get the inverted
selection in the main header when using a dark color with `$main-header`
but a light color with `--main-header`, which doesn't make much sense.
Not sure whether there's anything we can do about it.
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