Gitea Themes : import arc-green css

I’m trying some themes for gitea. And in one .css theme I found, there is this :

@import "/css/theme-arc-green.css";

Of course, there is no theme-arc-green.css file in my public/css/ folder. Where can I find this file ? (or find what to put in the @import line to import the base theme arc-green ?

I installed Gitea in Docker on my Synology NAS, mounting a volume like this :

       - "/volume1/docker/gitea/data:/data"
       - "/etc/localtime:/etc/localtime:ro"

So I put my custom css themes in :
And I’ve got this header.tmpl file in data/gitea//templates/custom/

<link rel="stylesheet" href="{{AppSubUrl}}/css/{{DefaultTheme}}.css">
<link rel="stylesheet" href="{{AppSubUrl}}/css/{{.SignedUser.Theme}}.css">

Thanks for your help.