The Button in 5.15.6 has a contrast ratio that is less than 3:1. A user with low vision might get affected as user won't be able to verify selected controls, which may impact the efficiency of the user.

To Reproduce: Create a Qt window and add a button and open accessibility insights. While in Insights select the Contrast screen (3rd button on the left side, looks like a paint tray) and select the button and then select the border of the button. Observer that the ratio is not 3:1
For Reference look at the photo(s) attached.

Related Bug: QTBUG-96950 Scroll Bar Contrast Ratio for border is less than 3:1 - Qt Bug Tracker