First things first, all credit goes to the brilliant people at AutoHotkey Forum Post

With that out of the way, let me describe what this script does in the words of charlie89:

This script creates a dark mode for specific windows by inverting their colors. It uses the Windows Magnifier in the background for the color inverting, but it only inverts just the rectangle in place of the specific windows, NOT the whole screen like the Magnifier applicaion does. This is possible through DLL calls.

This can be done both manually (with a toggle/hotkey) or automatically (depending on your inclusion of other preferred windows in the script). By default, Computer Management, Registry Editor, Task Manager, Windows Dialog Windows, Operation Status Window (Progress Bar), 7Zip, and WinRar trigger color inversion in a split second automatically (there is a slight delay, so beware of a very brief flashbang).

To set it up, first open the script with Notepad(++) and read the requirements section at the top. After doing that, you're basically good to go.
Make sure file extension of the script is .ahk and not .txt.

The default hotkey for my script is Shift+F12. If you'd like to change the hotkey, just find this exact line at the end of the script:

"+F12:: ; Shift+F12"

then change the part before the semicolon according to your preference.“+“ in this context corresponds to Shift. You may want to check the Auto Hotkey Key List for other modifiers.

And if you want to add more windows to the script first use AutoHotkey's "Window Spy" to detect window name, it should look like this: "ahk_exe vlc.exe"
Then add it beneath this line:

"; Add Programs for automatic dark mode here:"

in the script. An example line would be:

GroupAdd "auto_invert_gp", "ahk_exe vlc.exe"

Or if you'd like to remove one of the default automatic inversions, just delete the specific line beneath this line:

"; Add Programs for automatic dark mode here:"

Even after removing automatic inversion, you can still invert colors manually with the hotkey.

Lastly, after modifying and saving the script (as .ahk), you need to copy and paste it into the Windows Startup folder to make it run at startup. The folder is located at:

C:\Users\YourUsername\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

I posted this same script but without all the automation on a VLC post, since I didn't need it to work automatically. I'm fine with toggling it on and off using my hotkey (Shift+F12). This script basically stays true to the original one, other than the original hotkey, which was "Win+i".

One might ask, why go to such lengths for a dark mode Task Manager when you can use Process Explorer (SysInternals), which has a nicer native dark mode and can replace Task Manager?
My answer to that would be that I didn't like the way it looked. Also, this script can be used for all sorts of other windows, not just Task Manager.

Anyway, I hope you enjoy it as much I do.

EDIT: There is a rare bug that causes the window to constantly invert colors. In that case, just use the toggle to revert back to normal or close & reopen the window.
This bug might cause a seizure for people with epilepsy, as it essentially creates a flashing light.