The Snap sandbox prevents snapped desktop applications from accessing the themes of the host operating system to prevent compatibility issues. In order for Snaps to use a theme, it also needs to be packaged in a separate Snap. Many popular themes are packaged by the Snap developers but some themes are not supported yet and uncommon themes have to be installed manually. If a theme is not available as a Snap package, users have to resort to choosing the best matching theme available. Work is ongoing to make it easier for third parties to package themes in a Snap and to automatically install uncommon system themes.
