depending on your system theme. If set to 'custom'， the theme will be set to a random one. The random themes are not saved to your config. If set to 'favorite' only favorite themes will be randomized. If set to 'light' or 'dark'， After completing a test， custom themes will be randomized. ， only presets with light or dark background colors will be randomized， respectively. If set to 'auto' dark or light themes are used，。