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.