Improve error handling

- Add theme handling to error pages
- Center error message

share/jupyter/voila/templates/base/error.html

@@ -1,7 +1,17 @@
 {% extends "page.html" %}
 
+{% block stylesheets %}
+  {{ super() }}
+
+  <style>
+    .voila-error {
+      text-align: center;
+    }
+  </style>
+{% endblock %}
+
 {% block body %}
-  <div>
+  <div class="voila-error">
     <h1>{{ status_code }}: {{ status_message }}</h1>
 
     {% block error_detail %}

share/jupyter/voila/templates/lab/error.html

@@ -0,0 +1,12 @@
+{% extends "voila/templates/base/error.html" %}
+
+{% block stylesheets %}
+  {{ super() }}
+
+  <style>
+    body {
+      background-color: var(--jp-layout-color0);
+      color: var(--jp-ui-font-color0);
+    }
+  </style>
+{% endblock %}

voila/handler.py

@@ -23,7 +23,7 @@
 from ._version import __version__
 from .notebook_renderer import NotebookRenderer
 from .query_parameters_handler import QueryStringSocketHandler
-from .utils import ENV_VARIABLE
+from .utils import ENV_VARIABLE, create_include_assets_functions
 
 
 class VoilaHandler(JupyterHandler):
@@ -192,6 +192,30 @@ async def get(self, path=None):
             self.write(html)
             self.flush()
 
+    def render_template(self, name, **ns):
+        template_arg = (
+            self.get_argument("voila-template", self.voila_configuration.template)
+            if self.voila_configuration.allow_template_override == "YES"
+            else self.voila_configuration.template
+        )
+        theme_arg = (
+            self.get_argument("voila-theme", self.voila_configuration.theme)
+            if self.voila_configuration.allow_theme_override == "YES"
+            else self.voila_configuration.theme
+        )
+
+        ns = {
+            **ns,
+            **self.template_namespace,
+            **create_include_assets_functions(
+                template_arg, self.base_url
+            ),
+            "theme": theme_arg
+        }
+
+        template = self.get_template(name)
+        return template.render(**ns)
+
     def redirect_to_file(self, path):
         self.redirect(url_path_join(self.base_url, 'voila', 'files', path))