Hi,
You posted on Spanish SubForum.
Post on Android International to be readed and considerated.
--
About your question, don't know about any option related.
Also,
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.
Windows 7 (x64)
Opera Test profile | Opera Back up | Reset