For the moment
Blackout
Darken or blur everything except the window you're using.
Show me how
Turn it on
Keyboard
Press ⌃⌥⇧N. Press it again to turn it off.
Mouse
Click Blackout under the quick actions in the menu bar panel.
Options
The menu bar panel sets how dark it gets in dark and light mode: Dim, Dark or Darkest. You can also turn on Blur as well.
"Hold Fn to lift it" is on by default, so you can hold Fn to see everything for a moment.