The Fyne toolkit provides two themes to match a user's preference—a light theme and a dark theme. To change the theme for an application, the environment variable, FYNE_THEME, can be set to light or dark. If you are implementing a custom widget, it's advisable to test it with at least these two themes:
The default dark theme
The alternative light theme
At the time of writing, Fyne does not provide the ability to download user-created custom themes, but this may change in the future. It is, however, possible for an application to be displayed using its own theme. After implementing the fyne.Theme interface, you should pass an instance of the type to your application configuration using app.Settings().SetTheme().