Introduce a design system tokens file (`_tokens.scss`) based on the WordPress Design System, providing standardized values for colors, spacing, typography, border-radius, and elevation.
Update buttons to use CSS custom properties for theme color support across all admin color schemes. Restyle primary buttons with filled theme-color backgrounds, secondary buttons with outlined borders, and add a tertiary button mixin. Adopt Gutenberg-style focus rings with outset box-shadows and transparent outlines for Windows High Contrast mode compatibility.
Increase default input and select height from 30px to 40px, apply 2px border-radius, and update border and placeholder colors to match the design system. Restyle checkboxes and radios with filled theme-color checked states and white indicator marks, using outset double-ring focus styles consistent with Gutenberg.
Apply compact 32px sizing to list table controls, page-title-action buttons, and toolbar elements. Convert tag input layouts and the major-publishing-actions bar to flexbox.
Props fabiankaegy, joedolson, audrasjb, joen, phpbits, magaliechetrit, karmatosed.
Fixes#64547.
Built from https://develop.svn.wordpress.org/trunk@61645
git-svn-id: http://core.svn.wordpress.org/trunk@60956 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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