Correcting levels in dark renders I've been experimenting with Blender for a while and am starting to get things looking how I want, but the renders are coming out too dark.  What's supposed to be white is coming out as light grey.  Adding more lighting just results in the output becoming more washed out.  



(ignore the white outside the window, that's the result of losing the alpha channel when converting from PNG to JPEG)

The levels look like this in an image editor.  As you can see from the histogram the brightest colour is some way below white:



I can use the image editor to correct the levels to what I was expecting by dragging the right hand marker to the point where it's just to the right of the last peak.  This is what it looks like after correction (again, ignore the white outside the window, that's where the alpha channel was)



However I'd much rather have this done in Blender as I'd rather not have to manually correct every image it generates.  

I know Blender has a compositor, and that it can probably be used to do level correction, but I know very little about how to actually use it.  I've experimented a bit but haven't really found anything like what I've done in the image editor I've been using.  Basically what I could really do with is a node that says "Treat the lightest shade of grey in the image as if it was white and adjust all other colours in the image in proportion to that".  

Does something like that exist in Blender?  If so how can I apply it?  

I'm using Blender 2.82 with Eevee as the rendering engine and Freestyle to add lines.  As you can see I'm not going for photorealism.
You can access the color management panel in the render properties. Check "Use Curves" and then you can modify either the contrast or each color channel independently