You've wondered, "Which WordPress theme is that?" When you came to a website. Of course, not every website you see uses a WordPress theme, but because WordPress nowadays accounts for more than 25% of the websites, this is not so bad at all.
Of course, you can look at the source code of the website to see if a WordPress theme is used, but it can also be a lot easier. There are websites that are available for you. An example of this is What WordPress Theme is That?
All you have to do on the website is to fill in the URL of the website where you want to find the theme. Then click on the search icon and the information will be retrieved. If the website can not handle what the theme is, then this can be done by two things:
The website you filled in does not appear to be a WordPress website. Therefore, no WordPress theme can be found.
In this case, the website does use WordPress, but not a standard theme. This means that the website uses its own theme.
In some cases it will be indicated that the theme is a child theme. In that case, the theme is based on a (mostly purchased) WordPress theme, but tailored to its own requirements. If you can still find out which theme was used in this case, this will also be displayed. Please note that the original theme may look drastically different.
In addition to the WordPress theme, the website also looks at the installed WordPress plugins. Therefore, should there be some functionality available on a website, you can also find out if they use a particular plugin. Although they can not detect all plugins, the most popular WordPress plugins will be displayed.