Modify the Contrast Control settings for RFP

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1898096

Description

Changed the browser colours setting. I think it is basically the same as before, but removes an option to force colours without using a high contrast theme. But we should double check how this was implemented with respect to resist fingerprinting.

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