diff --git a/ckanext/pygments/helpers.py b/ckanext/pygments/helpers.py
index 28e3dc4..9f2f6fb 100644
--- a/ckanext/pygments/helpers.py
+++ b/ckanext/pygments/helpers.py
@@ -40,7 +40,7 @@ def pygment_preview(resource: dict[str, Any], theme: str) -> tuple[str, str]:
             else:
                 data = resp.text
 
-    css_styles = HtmlFormatter(style=theme).get_style_defs(".highlight")
+    css_styles = HtmlFormatter(style=theme or "default").get_style_defs(".highlight")
 
     return (
         highlight(
diff --git a/ckanext/pygments/templates/pygment_form.html b/ckanext/pygments/templates/pygment_form.html
index ea2eecd..aa14a7e 100644
--- a/ckanext/pygments/templates/pygment_form.html
+++ b/ckanext/pygments/templates/pygment_form.html
@@ -2,4 +2,4 @@
 
 {{ form.input('file_url', id='field-file_url', label=_('File URL'), placeholder=_('eg. http://example.com/test.sql  (if blank uses resource url)'), value=data.file_url, error=errors.file_url) }}
 
-{{ form.select('theme', id='field-theme', label=_('Preview theme'), options=h.get_preview_theme_options(), selected=data.theme, error=errors.theme) }}
+{{ form.select('theme', id='field-theme', label=_('Preview theme'), options=h.get_preview_theme_options(), selected=data.theme or "default", error=errors.theme) }}