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 (SPCSS) - Classic SharePoint. Depending on the SharePoint configuration you may install all the packages.
Sandbox Solution (Classic SharePoint) 📁
── yourthemename.SPCSS.wsp
Modern Solution (Modern SharePoint) 📁
── THEME_NAME.json
── THEME_NAME.spcolor
── THEME_NAME.spfx.sppkg
── Installer.ps1