Sub-directory site dashboard broken after enabling multi-site

I enabled multi-site on my WP blog which is about 8 months old and forced it to use sub-directories instead of sub-domains. Then I started adding more sites.

Now, it almost works except that the dashboards of all the sub-sites have no CSS or images (actually the sites themselves have no style from the Twenty Ten theme either). All the image and stylesheet URLs have the site slug in them rather than being located at the root of the network.
So they’re NETWORK_URL/site-slug/wp-admin/css… instead of NETWORK_URL/wp-admin/css…

I did try to make some changes to .htacccess to fix the above but it didn’t work.

Here’s the current .htaccess file.

# BEGIN WordPress
<IfModule mod_rewrite.c>
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]

#fix dashboard appearance
RewriteRule ^(.+)/wp-includes/(.*) ^wp-includes/$2 [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]
</IfModule>
# END WordPress

Any solutions?