There are two options to change screen style, if you have a visual impairment. Open Settings, select General, and tap Accessibility.

First, you have Invert Colors. If you enable this then all the colors are flipped. White becomes black, black becomes white etc.

The second option mutes a lot of the colors in the design. Enable Grayscale to tone down the whole look of iOS.

screen-options

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.