Below are the steps that need to be taken in order to apply Prime to the Site Master Page Setting drop-down menu under the Master page in Site settings.
1. Add the Look and Feel app from your Site Contents. (Note: There is a known issue regarding the timeout error while adding the app. If you receive a timeout error message, please retry until the app is successfully applied.)
2. Next, we need to add the Stylizer app from the Site Contents. Prime will now be available in the drop-down menu for the Master page settings
3. The Last step is to actually apply the Prime Master page setting. Go to Site Settings > Master page > Select Prime > Press Ok
Now your site will display the Prime Master page.