changed header searchbox border from none to var(--border)

[bisd]

I changed the setting --header-search-input-border: none to --header-search-input-border: var(--border) in variables.css. Hope that is ok?

[markconroy]

Note: this is a PR to fix the issue in #499

[finnlewis]

@markconroy you happy enough with this?

[finnlewis]

Discussing in Merge Monday.

This might place a border on existing websites search form, unless they have specified no border in the child theme.

I will test it and generate some screen shots.

[waako]

Transparency is our friend here, rather than having a default colour set from the --border variable.
Maybe introducing a new variable: --border-transparent: 1px solid, to be used in other places this crops up.

So we end up with --header-search-input-border: var(--border-transparent).

Header with the 1px transparent border, more or less invisible and shouldn't affect existing sites (where not overriden):

What that gives in Windows High Contrast Mode:

[bisd]

I have tried to implement what has been suggested by @waako. The branch has been updated.