A very valid question....I suppose you cut a slit through the scalp and skull and slide a light emitting ribbon in between the brain surface and the skull in the area of the tumor and light'er up.
https://www.photonics.com/Articles/Flexible_Glass_Substrates_Enable_Large-Scale/a63967
To some extent tissue does allow light to pass. (close your eyes and look at the sun, or see how much light from a flashlight can be seen through your hand.
If you need it over a large area I suppose you pull back a flap of scalp, get your bone saw out and remove enough skull to allow direct access to the brain surface and your light source.