Only include hljs css in mail html if if needed

src/routes/admin/mail/formatter.ts

@@ -41,11 +41,13 @@ type Mail = {
 };
 
 export function format(subject: string, content: string, user: User): Mail {
+	const html = md.render(content, {user});
+	const css = html.includes("hljs") ? `<style>${hljsCSS}</style>` : "";
 	return {
 		to: { name: user.name, address: user.email },
 		subject: replacePlaceholders(subject, user),
 		text: replacePlaceholders(content, user),
-		html: `<style>${hljsCSS}</style>${md.render(content, {user})}`
+		html: css + html
 	};
 }