Change wp-content without changing the name of the folder

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