WordPress support forum question:
What theme does this site use?
Here’s how I answer questions like this:
- Visit the site.
- Right click and select “View Source”.

- Press CTRL-F for a search box and type in “themes”.

- Look for a link to the theme.

It’s a safe bet that this site is using TwentySeventeen. But click the search arrows to verify!
In actuality, I’m using a child theme, sds_2017, but that’s obscured because I’m optimizing the HTML and CSS for the site, so a lot of stuff is hidden, aggregated, or obscured in the name of efficiency.