Why is custom.css not loading in VirtueMart modal windows?

Question

Joomla

In the custom.css template, it is connected like this:

if(JFile::exists(JUri::base(true) . 'templates/' . $this->template . '/custom.css')) {
    $doc->addStyleSheet($this->baseurl . '/templates/' . $this->template . '/custom.css');
}

There was a problem in the VM: it does not load into modal windows (ask a question, a pop-up cart window, etc.).
I tried to load it in the template without the existence check condition - to no avail.
How to fix?