base URL for external stylesheet ?

Hello,
This is my first message here and I’m a newbie on Codeigniter 4 (and I didn’t know Ci3).
Maybe I don't know a trick to deal with relative path in my stylesheet and I've got this problem :

I did an app on local (on root) and now I'm testing it online in a subfolder (« demo ») on my website. I’ve changed baseURL in config files (App.php and .env like : http://mydomain.com/demo/) and added the function baseURL() on all my hyperlinks. So, everything is ok for testing online, except...


Every URL in my external CSS sheets are wrong and and I can't change them with a php function. ( for exemple background : url('/img/arrow-top.svg'); should be background : url('demo/img/arrow-top.svg') ) Is there a way to fix that (with Ci4, html, htacess ?) without changing my CSS (and re-change them when I put my website on the root ?) 

I've tried to put a base in my header :
<base href="<?php echo base_url(); ?>/" target="_blank">
...but this only affect href on my pages... not the relative path inside my external stylesheet. 

Thank you in advance for any reply.