Theme

This is TwentyNineteen, an official theme made by the creators of WordPress. TwentyNineteen is also the default theme that will be installed with a fresh WordPress 5 installation. That may be the main reason, why this theme is very popular, because it is very basic and there are not many interesting features.

Using the TwentyNineteen WordPress Theme

Yes, it works in every aspect. TwentyNineteen is responsive, means mobile-friendly, and compatible with all known browsers. The WordPress default theme is also accessible and well-prepared for SEO. You can use it as is, as a parent theme or as a a starter theme. The last option may not be the best idea, as you are not able to get the improvements that come with updates without losing your changes. Making all changes in a child-theme might be the best option to create something individual with TwentyNineteen.