Strip the preceeding forward slashes from your rules:
Example (incorrect)
RewriteRule ^/css/(.*) /wp-content/themes/themename/css/$1 [QSA,L]
Example (correct)
RewriteRule ^css/(.*) wp-content/themes/themename/css/$1 [QSA,L]
All rules (correct)
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteRule ^/index\.php$ - [L]
RewriteRule ^css/(.*) wp-content/themes/themename/css/$1 [QSA,L]
RewriteRule ^js/(.*) wp-content/themes/themename/js/$1 [QSA,L]
RewriteRule ^img/(.*) wp-content/themes/themename/img/$1 [QSA,L]
RewriteRule ^font/(.*) wp-content/themes/themename/font/$1 [QSA,L]
RewriteRule ^plugins/(.*) wp-content/plugins/$1 [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress