displaymac promacosscreen-sharing
I have recently updated the OS in an old Mac Pro to El Capitan. Since then I have severe display problems. I must add that I access the system via screen sharing. The fonts are horribly blurred in menu bar drop-down menus, making them unreadable, and also in some packages the whole menu is diplaid in black. The first problem was sorted using the darkened menus and dock. But not the second problem.
This is probably not a problem with Screen Sharing, but a problem with iTeleport. Using screen sharing's default setup, I can see both of my iMac's displays from another computer:
And, it appears that iTeleport also supports multiple displays (see their FAQs).
I suggest that you check the connections to the extra display, and see if there are any settings for iTeleport.
You can also look at the Screen Sharing preferences by opening /System/Library/CoreServices/Screen Sharing.app and going to Screen Sharing-> Preferences. However, it doesn't appear that that allows you to change which display(s) are used.
I've occasionally had problems over the years with weird mouse issues, and bar once, they were all fixed by deleting the "com.apple.driver.AppleHIDMouse" file located in ~user\Library\Preferences. Upon reboot the file will be recreated, and you can bak it up first in case it changes something. Software updates can sometimes change/corrupt plist files, and deleting them to fix issues is fairly common.
Worth a try if you're struggling for a fix.
Best Answer
I think I've got the solution. Apart from enabling dark menu and dock all you have to do is to turn on "Increase contrast" option. See below: