feat: server side dashboard css for less repaint (#10850)

superset/templates/superset/dashboard.html

@@ -18,6 +18,15 @@
 #}
 {% extends "superset/basic.html" %}
 
+{% block head_css %}
+  {{ super() }}
+  {% if custom_css %}
+    <style class="CssEditor-css" type="text/css">
+      {{ custom_css }}
+    </style>
+  {% endif %}
+{% endblock %}
+
 {% block body %}
   <div id="app" data-bootstrap="{{ bootstrap_data }}" />
 {% endblock %}

superset/views/core.py

@@ -1700,6 +1700,7 @@ class Superset(BaseSupersetView):  # pylint: disable=too-many-public-methods
             entry="dashboard",
             standalone_mode=standalone_mode,
             title=dash.dashboard_title,
+            custom_css=dashboard_data.get("css"),
             bootstrap_data=json.dumps(
                 bootstrap_data, default=utils.pessimistic_json_iso_dttm_ser
             ),