HeyMargit. This is classic iPhone 6s backlight filter damage. No question.
You can do an out of warranty swap at Apple for $299 USD, or have a microsolderer change your backlight filter. It is a straightforward repair and quality shops offer no fix = no fee to look at it. We just fixed one from Norway last month. A shop called RepLab--they can give your more info since they've seen this exact issue before.
Don't blame the electrician--most of the stuff on the internet about how to change screens fails to stress how important disconnecting the battery first is before iPhone screen replacement. The 6s seems to be highly susceptible to backlight failure from simply pulling the screen while the device is still under power.
Sorry for your luck, but definitely repairable.