After some additional "googling about" and multiple peeks into my registry - I am beginning to question if the change can even be made.
Reference this link, albeit over a year old....
And this follow on:
Note this sentence from the first link.
"The latest version of the app, moves its name “Piriform Cleaner” from the center of the title bar, in v5.08, to the left corner of the title bar, right next to its icon. CCleaner 5.09’s release notes does specify that there are some improvements to the UI’s styling as well as speed, and that it adds the Windows app border. You can see the border by comparing the image featured above, and the one below."
Obviously from your viewpoint the styling is neither desired nor an improvement. And there is the mention of "Adds the Windows app border". Sounds permanent....
I also did a couple of searches using "Aero CCleaner Piriform" with a few variations.
Found a few other links and postings on the topic. A couple of those links were getting into technical details that included the registry and other configuration considerations. E.g., DWM, UxSms....
Here is one such link. Not sure if the link is fully applicable as you have not mentioned any "please wait" responses. But one posting therein indicated that that may not present.
What I have failed to determine thus far is whether or not Piriform/CCleaner has completely locked down any changes to its interface by either hardcoding the appearance or by ignoring any of the usual methods to change the styling to the user's preference.
Settings "greyed out" is not a promising sign.
Unfortunately, end user control is being eroded away for the sake of a "good user experience", "security", or other somewhat other contrived claims. Paying for software as a service; configuration as a service seems to be looming.... That would be a rant for some other venue.