This is not a problem with the map, it is a function of the Garmin GPS, it basically switches to Night Mode on or after the sun has set (it knows this as it receives this information from the satellites).
You can either live with it (it switches back to Day Mode, when the sun is up), or simply disable the Night function via the menus on the GPS. Please see the Garmin manual for your model for details on how to do this.
On most Garmin (handheld) devices, you will find the setting under, Setup -> Display -> Colors