Windows 7 has a built-in good looking dark mode
You can download a software like Iris, it has a smart inversion mode which will let you have a dark mode on anything you want and at the same time look better than simply inverting every single color.
Also, you will benefit from Iris main function, which is saving your eyes while at the computer
Iris reduced the blue light that your monitor emits so you can have an awesome sleep and worry less about it
More info on Night mode here: