@leocg On another PC, where I have to use Chrome, the pages are normal. Also, when I turn Opera dark mode off, it doesn't change.

Hello @noirsette.
As suggested, to solve the problem, I basically did a whole reset on my Opera installation, cleaning even the online synced data for my account. Only then the issue was over.

I recommend you taking a look and/or saving (with screen shots or something) all your relevant config, open tabs and etc, then doing the cleanup.

@leocg Will try. But how do I reset the synced data?

@leocg I don't know, but probably. But what setting should I change to make it stop?
Or if I should just reset the settings that are synchronized, how do I do that?

@leocg Yes.
I even tried a new whole "profile" in Opera. Renamed the "Opera GX Stable" profile folder and let opera create a new one. But then when I connected to my opera account, it went back.

@leocg On another PC, where I have to use Chrome, the pages are normal. Also, when I turn Opera dark mode off, it doesn't change.

@leocg Yes. On Opera and on Google. The thing is: when I turn Google Dark Mode off, it goes to an even worse dark mode.
For instance, Google Search with dark mode on:

And with Dark Mode off:

@rhiyaro Image attached (didn't upload for some reason):

Hello,
I'm having a problem related to Opera making Google pages into dark mode. Some google pages (at least are the only ones I noted this) are forced into Dark mode by Opera GX.
The problem started on another PC and even when I started using a new one, the problem persisted. I tried cleaning the profile folder of Opera in my PC and when I opened the pages, they were ok, but the problem returns as soon as I synchronize my data with my Opera account.
Also tried the "Reset to default" in Opera GX configurations. The pages return to normal, but, again, as soon as I log in to my Opera account, it bugs.

With that in mind, I'm pretty sure it's related to my opera account, but don't know how to solve, or what exactly is causing the issue.

Obs.: I'm not using, nor have activated the "Force Dark Pages" option.

An example page, of Google Docs.