There are two options to change screen style if you have a visual impairment. Open Settings, select Accessibility, and then Display & Text Size.
First, you have Classic Invert, which reverses the colors of the display. Perhaps more usefully, you can enable Smart Invert to reverse most of the interface colors but leave images and other media with their standard coloration.
You can also tap Color Filters to adjust the screen’s coloration. This is particularly useful for users with color blindness. You can choose a color to filter out, or enable Grayscale to tone down the whole look of iOS.
It’s also handy to know where these options are in case someone has enabled them by accident and you need to turn them off.