How can I change colors for the minimise / close / maximise controls in the titlebar on macOS? Is there any way to change the  colors? I have already disabled SIP and downloaded ThemeEngine, but it doesn't want to save my changes anyway. It says that I don't have enough permissions, even after chmod 777 command for SystemAppearance file. 

I am running macOS Mojave.
Make backup for safety


DISABLE " SIP " from recovery mode How do I disable System Integrity Protection (SIP) AKA &quot;rootless&quot; on macOs [OS X] and reboot to do rest of the steps, while SIP is disabled.
Download this app (latest release) https://github.com/alexzielenski/ThemeEngine/releases
Then follow this path      (/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/).
Just open Finder and in the top menu choose Go > Go to Folder and insert the path.
Now there are many .car files. You need to choose one of these (depending what you trying to edit.


SystemAppearance.car (It stands for any color scheme [blue, purple, pink, red, orange, yellow, green] EXCEPT GREY scheme. 
GraphiteAppearance.car (Only for grey scheme)
GraphiteDarkAppearance.car (For grey + dark theme)

Copy-paste chosen file 2 times, on different locations. First for modding and second for safety.
Open the file for modification (using ThemeEngine) and scroll down. Find the "Folder" with the circles. Use the filter "Scale" in the bottom of application. 
Use 1x icons for not-retina displays and 2x for retina. You can modify only "regular" circles, because you won't see small size circles probably anywhere.
Copy the pictures (drag it from app to desktop) you want to modify in the separate folders. You need this, because they will rename as they get together.
Open copies in any photo editor (I used Photoshop) and paint them as u wish. But don't forget to check the size. They mustn't be too big compare to the original, because they won't fit in some apps.
Once you finished - drag and drop modified files to their place. For example: Modified close button > Original close button. Replace all of the regular size pictures with your's.
Quit the app after saving your preset. Copy this modified file one more time for safety.
Drag and drop it in the original folder where you took it. Confirm all of the suggestions and REPLACE the original. 
Reboot into recovery mode and turn on SIP using csrutil enable.


Well done! You did it! Congratulations!

If you have any problems with step 11 - you probably haven't disabled SIP.

Original files and my preset (white buttons, black symbols):
https://drive.google.com/drive/folders/1SewDd32Pwll_O4iZvomL0y8xtBwz17Py?usp=sharing