@koyasakura
I don't think extensions are necessary since malware are rampant everywhere, but the extensions you mentioned will come in very handy by blocking ads, enhancing customization and improving dark mode. But there is risk of making Opera to allow extensions. The risk is making Opera to allow extensions will open the door for malicious extensions to enter Opera browser and invalidate the inbuilt security, privacy, and malware protection features of Opera. This is seen in the case of Google Chrome as stated in www.forbes.com.
Opera already has an ad blocker and dark mode so AdGuard and Dark Mode are not necessary.