This patch adjusts the Breeze SDDM theme to remove the semi-permanently-blurred background, which has not been especially popular.
Adequate background contrast for the avatar was already added in D16879.
The buttons get better contrast by using new variants that include contrasting background circles from D15999. If these icons are not available, it falls back to the old ones, which is important for 3rd-party themes that will be using the old names and generally have their own backgrounds to solve the previous contrast problem.
The bottom row of icons get adequate contrast by putting a dark translucent bar behind them.
The net result is that the background image is now actually visible. But this does not come at the expense of usability since all the UI elements now get adequate contrast no matter what the background is.
Depends on D15999