Is there any way of freezing the browser window intentionally like alert, confirm and prompt (in short: "acp") does?

I want to show a modal dialog with custom css instead of the native popups for "acp" but also want to have the ability to freeze the browser until I have users feedback just like "acp".

But man, why? This is bad practice (uh I have to downvote)!
So when it is bad practice - then why does "acp" actually offer this synchronous behavior? Because in some particular scenarios its just exactly the right tool for an appropriate UX. Those native modals do look so ugly and are also very limited at the same time.

Here's just one quick and dirty example where it would be totally fine to freeze the browser until the user gives feedback. Lets say we have a form with an input[type="reset"]-element. So before we really let the form reset we ask the user something like: "Are you sure you want to reset (data will be lost)?".

If I would be fine with the native modal look (which I'm not) I could do:

document.querySelector('form').addEventListener(
  'reset', e => confirm('Are you sure you want to reset (data will be lost)?') || e.preventDefault()
);

So "everybody" should agree (edit: or at least somebody could) this isn't bad practice, right?
But how can we achieve the same with a custom styled modal/dialog/popup?
I'm -of course- not asking for the HTML/CSS part but for the capability of freezing the browser window via JavaScript!

To be honest, I actually expect some downvotes for this but maybe there is this one Killer JavaScript Ninja, who have this one special hack to make it possible...