The themes are provided in 2 different packaging all following Microsoft guidelines:
For Classic Experience:
- Sandbox Solution, for Sandbox environments (*.wsp).
For Modern Experience:
- Modern Solution, for Modern SharePoint (*.sppkg).
After unzipping your Theme package, you will find two zip files, a SPFx package - Modern SharePoint and Sandbox Solution (SPO) - Classic SharePoint. Depending on the SharePoint configuration you may install all the packages.
Sandbox Solution (Classic SharePoint) 📁
── yourthemename.SP02013.wsp
Modern Solution (Modern SharePoint) 📁
── yourthemename.json
── yourthemename.spcolor
── yourthemename.spfx.sppkg
── Installer.ps1