In Plasma 5.21, the close button is now red, but some themes don't look good with it. The close button color already can be changed using the "negative text" option, but it affects the color of the hover effect too. A dedicated option for changing the color of the normal, hover and pressed states would fix this problem.
This was done intentionally so that the same color is used everywhere for close/destructive actions. If you don't like the color, just disable the outline in the Breeze Window Decoration settings. Adding an additional setitng to control just the color of the close button outline circle seems like over-configurability to me given that it is *already* configurable by changing the "negative text" color, sorry. :)
Understood, thanks. I thought the negative text was used by another element in the window and changing it could create UX issues.