Theme seems to disappear after child blog creation

I’m running WP in multisite format. When I create a child blog as a subfolder although the creation of the blog itself works, after the last step in the blog creation process the user should stay on the main site’s home page but the page itself doesn’t load the theme correctly. It’s as if the theme is completely absent apart from the text from the page itself.

Someone suggested I check the htaccess file is correct.
This my file contents:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

Any suggestions?

Thanks for your responses.