#64762 Use sass to generate admin toolbar colours

[johnbillion]

This extracts the admin toolbar styles into a mixin so a standalone admin-bar.css file can be generated for each colour scheme. Facilitates custom colour schemes registering their own admin toolbar CSS file too.

Trac ticket: https://core.trac.wordpress.org/ticket/64762

Use of AI Tools

Claude Code was used to initialise the separate admin bar color Sass files for each colour scheme. Worked ok-ish, I subsequently manually adjusted a lot of it.

[github-actions]

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Props johnbillion.

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[github-actions]

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