Some unintended action of mine caused the style of the source to change to one with a dark background. How do I restore the TexShop default?
If you open the TeXShop preferences pane, the top right settings on the first panel will allow you to set the text and background colours. Click on each coloured rectangle to choose the colours you want. The left hand rectangle sets the text colour, and the right hand one, the background colour.
The defaults are to have text black and background white, as in the image below.
Open a terminal and do this:
defaults delete TeXShop foreground_R defaults delete TeXShop foreground_G defaults delete TeXShop foreground_B defaults delete TeXShop background_R defaults delete TeXShop background_G defaults delete TeXShop background_B