Drupal does rewrite url but can't load css/themes I have an url rewriting issue with my drupal 7

I'm trying to rewrite for example index.php?q=categorie&amp;categorie=laars to categorie/laars.html

So I downloaded the default drupal 7 .htaccess file from https://code.google.com/p/drupalgooglecode/source/browse/trunk/.htaccess

Then in the section 

# Various rewrite rules.
&lt;IfModule mod_rewrite.c&gt;
&lt;/IfModule&gt;


I added: 

RewriteRule ^categorie/([^/]*)\.html$ index.php?q=categorie&amp;categorie=$1 [L]


Above the default (I also tried below but that didn't rewrite the url):

RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]


But it doesn't seem to work. When the rewrite rule is added, my site's layout is all messed up. As if the themes/css is not loaded anymore.
The url does rewrite as I want, but it looks like drupal doesn't know where the theme files are. What may be the cause of this?

Thanks!
Stece

UPDATE: is it possible that url-rewriting of .htaccess conflicts with the drupal clean-url's module? Because theming that doesn't load, feels like something is conflicting. Not sure...

UPDATE2: also tried using a custom module with the hook_url_inbound_alter, but that's not working with me either (https://stackoverflow.com/questions/17176589/drupal-hook-url-inbound-alter-not-altering-my-url). That question is on stackoverflow, for some reason I hoped to get a quicker solution their.
Maybe I am missing something, but you shouldn't need the .htaccess rewrite.

It is likely that the 'conflict' is it rewriting the path to your styles. View the source and check what happens when you try to navigate to your css etc.

To achieve this effect all you should need to do (unless I am missing something) is to enable clean URLs, this will remove the index.php?q=. 

Then to add .html go to admin/config/search/path/patterns (with pathauto installed) and add .html after [node:title] or [term:name], whichever you are using.

So your patterns would look like:
[node:title].html
and
[term:name].html