Use system-colors if supported

[nuxodin]

The corresponding spec change: https://github.com/whatwg/html/commit/22154a4c1d426dddf187fb12c05fbcb8816c94a5

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.

[pooya121]

It does not have proper browser support. defeats the purpose of a polyfill

[nuxodin]

It does what a native dialogue would do on supported browsers, with a fallback for those that are not supported.

Is your objection that the polyfill does not work the same everywhere?