From its api docs:
This method allows Plasma to enable and disable the background
contrast effect for a given theme, improving readability. The
value is read from the "enabled" key in the "ContrastEffect"
group in the Theme's metadata file.
The configuration in the metadata.desktop file of the theme
could look like this (for a lighter background):
[ContrastEffect]
enabled=true
contrast=0.45
intensity=0.45
saturation=1.7
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