If you want to cover parts outside the triangle (either completely or slightly, for fog), you could draw a single shape that covers the entire window/scene except the triangle. It could be black for darkness, white with alpha for fogs or anything else really!
Another way is to use a lighting texture system, which can be a little more involved.
Basically, setup a render texture the same size as the window, clear it a solid colour (black or grey) and draw the triangle as a solid full white.
Then draw that render texture onto the fully drawn window using blend multiply. White parts are fully shown, black parts are erased and become fully black, greys are somewhere inbetween; they will look darkened.
A third way is the same as the second except it's the opposite approach.
Same render texture but clear it as white (or grey) and draw the triangle as a solid full black.
When drawing the render texture onto the fully drawn window, use blend add. Black parts are left as normal and white parts become fully white. Greys will bring up the colours towards white.
The first method is simplest and easiest and also really flexible.
The second can only darken, not add white fog.
The third can only add white fog, not darken.
