add second rewrite rule and how to construct?

I never wrote .htaccess before.

this is from wordpress pretty permalinks

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

this works

now i want to add another rule
to rewrite everything else:
/blog/wp-content/themes/themename/ (all folders and files)
also to /blog/ (all folder and files)

example: http://mydomain.com/blog/images/myimage.png from
http://mydomain.com/blog/wp-content/themes/themename/images/myimage.png
http://mydomain.com/blog/js/myjavascript.js from
http://mydomain.com/blog/wp-content/themes/themename/js/myjavascript.js

how do I add another rule?
and how do i construct the second rewrite rule ?

this is as far i got with a foolish attempt, but didnt work offcourse
RewriteRule ^blog/(.*)$ blog/wp-content/themes/themename/$1 [NC,L]