Fix theme consistency, i18n completeness, CSS variable naming, and hardcoded strings

src/app/admin/vouchers/page.tsx

@@ -92,7 +92,7 @@ export default async function AdminVouchers() {
 
       <form action={createVoucher} className="admin-card mb-6">
         <h3 style={{ marginTop: 0 }}>{t("newVoucher")}</h3>
-        <div className="grid cols-2" style={{ marginBottom: "0.75rem" }}>
+        <div className="grid grid-cols-1 gap-4 md:grid-cols-2" style={{ marginBottom: "0.75rem" }}>
           <label style={{ display: "flex", flexDirection: "column", gap: "0.3rem" }}>
             <span className="text-sm text-gray-500 dark:text-gray-400">{t("form.code")}</span>
             <input name="code" placeholder={t("form.codePlaceholder")} maxLength={255} required />