What is the default theme name in Ubuntu 16.04?
Solution 1:
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 light-themes.