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 =)