For those using Opera Mini 4.x on their old phones, there is no option to adjust the brightness level.
Opera mini can help them by having one of the following quick solutions.
1. Add "brightness" setting in either regular setting or in config: screen
2. Create a "night" skin with say 50% brightness which the user can switch-to during nights when the lights are switched off.
I am sure that it will help a lot of users who want to keep their old phone and have to rely on Opera Mini.
(P.S. Sent from my Nokia 6233 ;-))