@leocg no I have fixed the colours in opera browser, they were displaying the same as the screenshot. It is the problem/solution with MIUI not Opera.

@ucuncuulus I don't know if the redmi settings are the same as my Poco X3 NFC but here is what I did.

1. click on settings
2. type at the top dark mode
   3)then on dark mode page you will see 'Individual apps' click on that
3. Third Party App, scroll down until you see Opera or any app that is not displayed properly. Boom tis- Dark mode now displayed.
   Hope this helps!