unless you screen is OLED, dark-mode has no or zero impact on battery usage, maybe even non-significant impact on OLEDs
HP Envy does have OLED screen options. I'd assume it's what they have, if they thought dark mode was relevant.
Don't most LCD screens have localised dimming of the backlight these days?
HP Envy does have OLED screen options. I'd assume it's what they have, if they thought dark mode was relevant.