the same happens to me, it doesnt work. it is on 0 by default, and if you change it, it will still be 0 after saving the settings.
i found out that in the database, in the rl_configuration field, this setting is saved as an integer (i:0). changing it in the db to “d:0.5” seems to work (at least in the settings page you can see the number) but the page opacity still doestn work.
besides that, it is a great plugin =)
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