Please make certain you have installed the themes you want to use in the /themes directory.

To use a theme for a site, execute Hugo with the following parameter:

hugo -t ThemeName

or add this line to your site configuration:

theme:                      "ThemeName"

The ThemeName must match the name of the directory inside /themes.

Hugo will then apply the theme first, then apply anything that is in the local directory. To learn more, go to customizing themes.