The option does exist - but it is in opera://flags

Right clicking? I just left click, a pop-up message asks me to allow Flash and I click "Allow".

Opera can be set to "Prefer HTML" or "Prefer text", but if a message has only an HTML part you can't force it to be converted to text.

Looks like you'll have to edit a settings file to remove that "Always load images from (email address)", as it isn't part of Contacts. Let me look into it ...

Extension, though? I've seen protocol handlers (see opera://settings/handlers to disable that one), but Opera only allows extensions through its store for security reasons.

Looks like it is supposed to do that. When I first arrived at the site, the background was white. Clicking that made it black, then again made it the pink in your photo - and one more time made it white again. So it seems to be a "theme" button and they have 3 themes to choose from. Ideally it should remember the theme you've selected (until you clear cookies) and show you the same theme next time you visit.

Yeah, when I click the English button, the one you point to is translated as "Color".

Do you use Facebook? Every time you see a Like button, they are spying on you. Google spies on everyone, etc. I'm sure you know this.

Version of Opera? Are you logged in, and it behaves differently when you are? (If so, I'd need to know where the log in button is, as I can't read it.)

I tried about a dozen links in various areas of the page, the background never turned black - but I am using Developer (the latest unstable build) and possibly the issue is only in Stable or maybe Beta,

Any extensions that might be doing that?

Which link?

Opera Mail uses the Presto rendering engine that was in Opera browser versions 7-12; they can't make Mail open-source without also turning over Presto.

Then you'll have to explain what you mean by "old hidtory".