Accent-color theming

CSS `accent-color` and `caret-color` properties tint native user-agent controls (checkboxes, radios, the text caret) to a site's theme without replacing them, keeping the browser's built-in accessibility semantics intact.

grounded in: commits 7c78dc1 'Theme native form controls to the phosphor accent' and 14ab261 'Theme the native text caret to phosphor green'