I recently upgraded to BiaB 2021 for Mac (latest build) and when I open the style picker the usual red 'x' is missing so I'm unable to close the window. Some of the other windows have a similar problem.

Do you mean the three LEDS in the top, left side of the screen?

Mac screens do not use the red "X" to exit a screen.

For the Style Picker and other windows, instead of the red exit button at the top, use the 'Cancel" button at the bottom. This will exit the Style Picker window without changing the previously loaded Style.