On a submitted bug report I was asked whether I am using the default theme of Ubuntu 16.04. However, I could not find any information about the name of the default theme.
The default theme is called Ambiance.
To find this out, press Super to open the Dash and search for "Appearance". Click on the "Theme" dropdown box and you will see an entry called "Ambiance (default)".
If you're filing bug reports, you might need to know the package name containing the theme. The package name of Ambiance (and its light counterpart Radiance) theme is