I'm trying to rewrite multiple directories for my app using .htaccess

All requests are processed via the 'app.php' file in my 'Web' directory which works absolutely fine but I've got my assets (CSS/JS/Images) in the 'Assets' folder but I want to be able to access them by pointing to 'css/filename.css'. The 'Assets' folder is outside the 'Web' directory

If I try adding a anything to the root .htaccess telling the server rewrite requests with a '.css' or '.js' extension to the appropriate location in the 'Assets' folder, then the rewrite that sends all other requests to the 'app.php' file in the 'Web' directory stops working altogether.

Requests:
Code:
http://localhost/page -> /htdocs/Web/app.php
http://localhost/css/filename.css -> /htdocs/Assets/CSS/filename.css
http://localhost/js/filename.js -> /htdocs/Assets/JS/filename.js
Structure:
Code:
htdocs
- .htaccess

- Assets
-- CSS
--- file.css
-- Images
-- JS

- Web
-- .htaccess
-- app.php
/htdocs/.htaccess:
Code:
<IfModule mod_rewrite.c>
    #CSS
    RewriteCond %{REQUEST_URI} \.(css)$
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^css/(.*) Assets/CSS/$1 [L]

    RewriteRule .? - [S=2]

    #Reroute to app
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^$ Web/ [L]
    RewriteRule (.*) Web/$1 [L]
</IfModule>
/htdocs/Web/.htaccess
Code:
<IfModule mod_rewrite.c>
    DirectoryIndex app.php

    RewriteEngine On

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d 
    RewriteCond %{REQUEST_URI} !\.(css)$
    RewriteRule ^(.*)$ app.php?$1 [PT,L]
</IfModule>
Thank you very much for your help!