.Lots of designs work indoors. Great for winter in Chicago. Not so good for Bondi Beach. If you are putting in the work to have multiple theme variations for different devices and the visually impaired, you can offer a variation of a visually impaired theme as an extreme contrast theme for use in the sun.
The right colour choices will also work underwater on the waterproof smartphones. I am collating some notes in the linked page. What is your experience of Web browsing in extreme conditions?
Most screens are old fashioned LCD screens and they wash out in the sun due to the many layers of junk in the screen. Modern OLED screens have fewer layers, work better, and still wash out in the very bright light at the beach. Increase contrast and reduce resolution to cut through the sunlight.