You posted on Spanish SubForum.
Post on Android International to be readed and considerated.
About your question, don't know about any option related.
The new dark background has been added after long time and even Kiwi's dark mode is better than Opera's.
"Off topic Tip"
Follow the Signature's BackUp | Reset link.
Take the opportunity to start a Backup plan, even create a Template Profile.
SyncBack | Synkron helps.
Windows 7 (x64)
Opera Test profile | Opera Back up | Reset