Since it's just HTML+Javascript and all client-side, yes you can. The source code isn't provided, but you can save the page, the four JS files that make up the bulk of it, and the AngularJS libraries it links to and run it offline. You can find links to all the JS files in the source code of the HTML page.
I'm curious, what tools do you use for pixel art that you need an external viewer to preview it better? I thought every pixel art program had zoom? And many offer the ability to change the background colour.
If you're just looking for a quick way to view pixel art on Windows, Windows Image Viewer zooms without blurring, I use it as my default PNG viewer. On Windows 10 it requires some extra work to "unlock" and isn't available by default, but it's an option.