Humminbird 400TX Operation Manual page 30

Options. The Options Menu is a series of linked menu functions used
initially to set your preferences Once your preferences are selected, it is
unlikely these functions need to be accessed during the normal
operation of the product.
There are eight Option menus: Contrast, Units, Fish ID, Numeral size,
Transducer, Display Speed, Bottom Black, and Reset. Press the view
key at any time to exit options and return to normal operation.
Contrast. The Contrast function allows the user to control the level of
contrast of the LCD display. The 400TX will automatically adjust the
display contrast to compensate for changing ambient temperatures;
however, some situations may occur where manual adjustment will
provide a higher level of contrast.
Eleven contrast settings, +5 to -5, are possible. The display contrast will
change as the adjustment is made so the optimum level of contrast can
easily be adjusted. Use the ARROW buttons to highlight the desired
selection.