desktoppackage: make sure the accent color conforms to WCAG contrast criterion
This makes the visual presentation of the accent color and the background color of Plasma components have enough contrast ratio by following WCAG AA standard.
Though the criterion is for text color and background color, the value 4.5 still applies here. 3 wallpapers provided by the bug reporter were tested.
See also: WCAG 2.1 - Success Criterion 1.4.3 Contrast (Minimum)
BUG: 457861 FIXED-IN: 5.26