The reason the default pop ups in chrome don't look good is because they aren't really important, so there's no reason for the developers at Google to spend a bunch of time designing and building beautiful ones. If you look at the pop ups in most other browsers (to the best of my knowledge), they will look similar.
Your second screenshot looks like some sort of modified version (possibly bootstrap?) and has absolutely nothing to do with the default pop up.
If you want to change the look, you can't. It's part of the browser, not part of the website. But, if you really need to have a better looking one (and I would strongly recommend you look into different options as pop ups are bad UX), http://jqueryui.com/dialog/ will be able to help.