system.css: use only variables.css tokens, remove undefined custom properties

Replace the two undefined variables that had crept in:
- var(--admin-border) → #555  (in .log-output border)
- var(--admin-text-muted) → #969696  (inline style on log toolbar label,
  in both system.php and system-fragment.php)

Revert the incorrect intermediate attempt that mapped dark-UI hex values
to light-theme tokens (--bg-primary: #fff, --border-primary: #ddd, etc.)
and also revert the .admin-body override block that was added to
variables.css — variables.css is shared and must not have per-component
overrides.

All remaining var() calls in system.css now reference tokens that exist
in variables.css:
  --accent-primary, --accent-green, --error, --warning, --success,
  --text-tertiary
The dark surface colours (#1a1a1a, #242424, #0d0d0d, #555, #969696, etc.)
stay as literal hex values, consistent with how admin.css handles them.

public/admin/system-fragment.php

@@ -173,7 +173,7 @@ if ($tab === 'nginx_config') {
     }
     ?>
 <div class="log-toolbar">
-    <label for="lines-select" style="font-size:.84rem;color:var(--admin-text-muted);">Afficher</label>
+    <label for="lines-select" style="font-size:.84rem;color:var(--text-secondary);">Afficher</label>
     <select id="lines-select" aria-label="Nombre de lignes">
         <?php foreach (ALLOWED_LINES_FRAG as $opt): ?>
         <option value="<?= $opt ?>" <?= $opt === $n ? 'selected' : '' ?>><?= $opt ?> dernières lignes</option>