Inside your theme's SandboxSolution folder you will find a .wsp file which you will be using for installing your theme.
- Open your SharePoint Site;
- Click on Settings ⚙️ and then on Site settings;
Under "Web Designer Galleries", click on Solutions;
Note: If you're not working on your root site this option will not appear.
Click on Upload Solution;
- Upload the "yourthemename".SP2019.wsp file. You can find the file by opening your theme's pack and the SandboxSolution folder;
- Click OK;
- 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.
- Open the Settings menu and click on Site Settings;
Under Look and Feel, click on Master page.
Choose the master page you want to apply. Your theme's master pages starts by yourthemename;
4. Click Ok.
Master Page set! ✅