default_admin: Check if certain colors can an be built from variables to allow easier overrides

Follow up to #3606571: Round 2 of consolidating CSS Variables , where @larowlan asks if some of the colors in core/themes/default_admin/css/_variables/variables-color-semantic.pcss.css can be built from variables to allow easier overrides .

 /*
   * Status — each family has three roles:
   *   --admin-color-{status}       solid — icons, borders, emphasis on a neutral bg.
   *   --admin-color-bg-{status}    the light tint behind highlighted rows / reports.
   *   --admin-color-text-{status}  text ON that tint (AA-verified pair).
   */
  --admin-color-error: light-dark(#cc3d3d, #ce6060);
  --admin-color-bg-error: color-mix(in srgb, #de7560 15%, transparent);
  --admin-color-text-error: light-dark(#cc3d3d, #e69e9e);
  --admin-color-warning: light-dark(#d8b234, #dec15f);
  --admin-color-bg-warning: color-mix(in srgb, #e29700 15%, transparent);
  --admin-color-text-warning: light-dark(#826b1f, #e8d185);
  --admin-color-success: light-dark(#058260, #32cea4);
  --admin-color-bg-success: light-dark(#26a76930, #26a76940);
  --admin-color-text-success: light-dark(#1d6844, #8bd3b1);

It may or may not make sense to do so, and should be investigated.