Steps to replicate:

1. Export a space to html (choose the default theme)
2. Unzip the package
3. Check /styles/site.css

The top displays:
/* Could not locate resource: /includes/css/master.css */
/* Could not locate resource: /includes/css/wiki-content.css */
/* Could not locate resource: /includes/css/tabs.css */
/* Could not locate resource: /includes/css/menu.css */
/* Could not locate resource: /includes/css/menu-ie.css */
/* Could not locate resource: /includes/css/tables.css */
/* Could not locate resource: /includes/css/panels.css */
/* Could not locate resource: /includes/css/master-ie.css */
/* Could not locate resource: /includes/css/renderer-macros.css */
/* Could not locate resource: /includes/css/content-types.css */
/* Could not locate resource: /includes/css/information-macros.css */
/* Could not locate resource: /includes/css/layout-macros.css */

The workaround is to include the content of these manually, from the confluence distribution.