The most up-to-date docs on installing a Gatsby theme can be found on the Gatsby website, but here's a summary using this theme as an example:
yarn add @mlent/gatsby-theme-help-center
# or
npm install --save @mlent/gatsby-theme-help-center
Then edit your gatsby-config.js
file:
module.exports = {
plugins: [
{
resolve: '@mlent/gatsby-theme-help-center',
options: {
basePath: '/'
}
}
]
};