Creating Header-input class for dark header inputs

[jonrohan]

Fixes #1481

I added a Header-input class to make search boxes in the header dark.

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Latest commit: 4715037

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@primer/css	Minor

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[jonrohan]

@simurai These were the variables used in dotcom, but I think they're slated to be deprecated. Should we add to https://github.com/primer/primitives/blob/d607f36c4c974017d6974382f9bb88d6418211b8/data/colors_v2/vars/component_dark.ts#L23-L26 a border and inset variable? We could also have something for buttons maybe?

[simurai]

Yeah, there used to be the globalNav colors, but not sure where they were used.

I think this PR is good as is, but I guess we should move the header-search variables that are currently in "product" to "component". Now that the component supports the input.

[simurai]

Update: Moved in primer/primitives#181

[simurai]

Yeah, there used to be the globalNav colors, but not sure where they were used.

I think this PR is good as is, but I guess we should move the header-search variables that are currently in "product" to "component". Now that the component supports the input.

[simurai]

Ohh.. one more thing... does this input need a placeholder text color that is a bit dimmed? Currently on dotcom we use color: rgba(255, 255, 255, 0.75);.