Copy & past the following code to a /code element in your Custom CSS power-Up:

@import URL("https://justtolook.github.io/remnote-theme/clean.css");

The theme is using the font 'Montserrat'. If you don't have the font installed, you can get it at https://fonts.google.com/specimen/Montserrat?preview.text_type=custom. After the installation you have to restart RemNote to see the new font.