Fix docs banner (#6861)

Co-authored-by: Ben Sherman <bentshermann@gmail.com>

Author: christopher-hakkaart

docs/_templates/layout.html

@@ -1,8 +1,8 @@
 {% extends "!layout.html" %}
 
 {% block document %}
-    <div class="admonition site-banner">
-        <p class="admonition-title">Nextflow documentation is migrating</p>
+    <div class="admonition site-banner" style="background: #E2F7F3; border-left: none;">
+        <p class="admonition-title" style="background: #0CAE8E;">Nextflow documentation is migrating</p>
         <p>Nextflow documentation is being migrated to <a href="https://docs.seqera.io/nextflow/">docs.seqera.io/nextflow</a>. This site will remain available throughout the migration.</p>
     </div>
     {{ super() }}