Note that Task Manager is incorrectly scaled by Windows (DPI system aware), as I changed DPI scaling in the very beginning of this 'quest' and didn't restart PC for 62 days and 22 hours + But this all things are out of the scope of this question. Of course, me being crazy perfectionist, I went on the Windows 10 improvement spree along with the Dark Mode, like removing useless context menu items for Explorer, adding useful context items, making custom garbage collector for Trash Bin, etc. So here is my custom userChrome.css code where I implemented a perfect titlebar buttons. For Chromium based browsers it works fine, but for Firefox I found the solution lacking. Your browser title bar and/or buttons will get messed up by the custom theme.As this program, combined with my theme fixes, removes the Control Panel bugs. It's very important to install OldNewExplorer (you can find a link in the guide).As you can imagine, this took huge amount of time as I had 0 experience with theaming Windows and also general lack of documentation and information online. Also one major bug was that Control Panel and some few other places had nasty white patches in them. There was major issues with messed up minimise/maximise/close buttons. I used BIB3 theme, you can browse Deviantart or other resources for any dark theme you want, but BIB3 seems to work great for my goal of total Dark Mode.It is not possible to achieve adequate Dark Mode on unpatched system, I'm afraid ( there is one attempt at this, using high-contrast theme, but due to very poor Microsoft's High Contrast implementation, its very limited and almost unusable). This is only possible with theme patch, however. And it probably will look better than Windows 11. At first I thought it's not possible, but it is. Well, now I can give myself and others a definitive answer of how to do it.

I went on a few tangents, however, like learning a bit of assembly and injection into other processes, a bit of JavaScript/CSS to style Firefox too, made custom low level API registry access library, etc. I was focused on this all this time, almost all day long. I can't believe that 2 months actually passed since I embarked on the quest to make Windows 10 100% Dark Mode, in every window in the system.

Black green windows 10 theme