Concatenate strings in CSS content: URL()

Question

using css var, set image src

<img alt='no_img' /> 
<style>
:root {
          --myImgSrc: url("https://example/bg.jpg");
        }

img.someClass
{
 content: var(--myImgSrc);
}
</style>

now I want to change host, and concatenate value from param and path to img

 :root {
        --domain:"https://example/";
       }

  img.someClass
    {
     content: url(var(--domain) 'bg.jpg')
    }

Property is invalid