To increase the readability of your watch screen while exercising or navigating, you can change between light and dark themes.
With the light theme, the display background is light and the numbers dark.
With the dark theme, the contrast is reverse, with the background dark and the numbers light.
The theme is a global setting that you can change in your watch from any of your sport mode options.
To change display theme:
Switch between Light and Dark by swiping up or down or by pressing the upper and lower buttons, and accept with the middle button.