Just purchased a Vivofit Jr3 and also complained of screen brightness with or without backlight. These do NOT display the same brightness levels as are shown on all of Garmin's advertising photos/videos, and should be considered false advertising.
It appears that this has been an issue in several devices for several years now.
Without a solution to change contrast, background or increase brightness it is a disappointment for as good a company as Garmin (at least I thought).