We just installed Premiere Pro CC 2018 to test our extension in the latest version. One immediately noticeable difference was that the background of the Extension's content is pure-white. After some investigating, we found that the default background-color was transparent (alpha = 0), allowing the panel's backing color to shine through. This worked in previous versions of Premiere Pro. It was nice because the user's color settings in the panel were handled automatically.
With CC 2018, all extensions will now need some sort of color handling of their own, akin to what the PProPanel does, including theme change event handling.
It would be preferable to have the background color be the same as the actual application's theme color as in previous application versions.