It sounds like you are doing something unusual.
Screenshots are not normally "lightened" in any way -- they are taken as is.
There is a color enhancing tool in SC where you could increase the gamma or contrast, etc. if you want -- no reason it should be applying to center unless you are applying a special effect that is designed specifically for that.
what you *might* be describing with the apparent "darkness" is a smoothing/blurring view that may be applied when the screenshot is being shown at below 100% magnitude (it doesn't effect the actual image).
Next time you compare screenshots and think SC image is dark, click the 100% zoom and see if its fixed and normal after changing to 100% zoom.
If so, you can turn off the slight smoothing/blurring effect used when resizing from here:
Again, screenshots are simply not brightened or darkened by screenshot taking programs as a rule -- they are left as is. Whatever darkness you are seeing has to be just an artifact of some display mode, etc.
Let me know what you discover.