As stated in the previous page, I made it default by editing the files by hand. Still trying to make it open new links in new tabs when Opera is already running, instead of just coming to front without loading anything (it loads the page when the browser is closed prior to the click on the link). The stuff in the thread you mentioned seems to partly solve this issue (partly only, plus I'd prefer new tab instead of new window), but there are some drawbacks.

I wonder if adding "-newtab" before the "%U" would work. I guess it would be too simple. :P