The background for Nyx, must be a 1280 x 720 32-bit BMP. Alpha blending is taken into account. For that reason, if a solid background is required, that value must be 0xFF. There are sites that can produce the correct bmp.
The icons supported are 192 x 192 32-bit BMP. You can utilize transparency at will and make nice mixes with the button background.
Additionally, using the icon={sd path}, the icon will get colorized if the name ends in _hue.bmp. That only works nicely if the icon is a white layout with transparency.
The default system icons (icon_switch.bmp and icon_payload.bmp) can be replaced with white layouts that have transparency. They can also be replaced with normal icons if the following exist: icon_switch_custom.bmp or/and icon_payload_custom.bmp
The background must go to /bootloader/res/background.bmp
The icons can be utilized either via [boot entries names] -> boot entries names.bmp or by using icon={sd path}, which should point at a bmp.