Thank you very much for your response…
I confirm that I have the same issues wth the parent theme (I’ve switched back to it) …
Please check again my demo site, where I’ve posted a new demo post…
As requested I’ve reset the background color to the default color, however please note that the strange scaling of small thumbnail (featured) images occurs merely by activating the fourteen extended plugin (I didn’t check or amend anything within the customizing sections of the plugin).
The strange behaviour also doesn’t have anything to do with the background color settings (I’ve mentioned it merely by example).
“The behaviour occours upon the following events”:
(1) activation of the plugin
(2) amendment of “any” (other) of the customizing sections (this can be the backgrond color, header image, header color, featured content, etc.)
(3) saving the customizing settings.
Please note also that I’ve posted this post without amending anything using the theme customizer.
Thereafter I’ve reset the background color to default using the customizer.
=> In each case same result.