Added duplicates of system.svgz' icons' with added background.
The background color is hardcoded atm. Update: I set the background color to 0.9 #232627 and edited the color scheme of the files. This way a future update of the lockscreen can overwrite the background color now, while the file now displays the correct, dark background.
Picture of how the icons look in various situations:
FEATURE: 398741
FEATURE: 398742
FIXED-IN: 5.52