Dark photos and brightness control

My photos on my iPhone are all very dark and sometimes the darkness is on my phone on all parts of my phone so much so that when it is happening I can just about see who’s calling me. I have my iPhone set to the dark mode so the text is in white and the background is black so it saves power how do I turn the brightness up and make it stay there. I have brightness in my quick menu as I pull down from the top right of my screen but it doesn’t stay at the brightness I change it to

thanks