Thanks for getting in touch. It does sound like the theme on the phone is providing the dark background/font. You can try changing the theme to see if this gives a better viewing experience when your sister goes into settings or any other page.
Hope this helps.
No worries, @marc-knuckle. I don't think it's anything else as some themes do have a darker background and if one of these is applied, it'll need changing to something less restraining on the eyes. There won't be any other setting that will affect the colour or presentation of the background. Do let us know how your sister gets on with this and if she needs any other help, just give us a shout.