Inside your theme's SandboxSolution folder you will find a .wsp file which you will be using for installing your theme.

  1. Open your SharePoint Site;
  2. Click on Settings ⚙️ and then on Site settings;

  3. Under "Web Designer Galleries", click on Solutions;

    Note: If you're not working on your root site this option will not appear.

  4. Click on Upload Solution;

  5. Upload the "yourthemename".SP2019.wsp file. You can find the file by opening your theme's pack and the SandboxSolution folder;
  6. Click OK;
  7. Click on "Activate" to activate the theme.

Theme installed! ✅

Set the master page

Final step is to change your current master page to one of the theme's master page.

  1. Open the Settings menu and click on Site Settings;

  2. Under Look and Feel, click on Master page.

    changethemasterpage_1

  3. Choose the master page you want to apply. Your theme's master pages starts by yourthemename;

    changethemasterpage_3 4. Click Ok.

Master Page set! ✅