Opera 12.x and Chromium-based Opera till now have not offered reset browser settings button, the latest update for Opera 51 has added that.

Now when you visit Opera menu > Settings > Browser, you’ll notice newly added ‘Reset Opera’ section (under ‘Default browser) with ‘Reset browser Settings’ button. Clicking on that will revert your search engine in Opera to their default and resets browser settings.

Doing this also disable all extensions, removes cookies, pinned tabs, but your bookmarks, history, and saved passwords will not be cleared.

Resetting Opera 12.x  Settings to its default

Reset Vivaldi Settings

How to Repair or Reset Microsoft Edge browser

Chrome 29 Gets Reset Button, Know When to Use It