Custom plasma theme modification to panel-background.svgz shows in system settings but does not appear to be utilized to create create system panel background and text colors:
Note that panel-colorizer can change the background color but there does not appear to be a provision to change the text/icon color to contrast the background. Colorizer Icon/text shows system tray components but “add” only opens up some white space under the icon, but no prompt to enter specification for icon/text color. Panel colorizer appearance button does not show a configuration for icon/text color.