AKA how to make opera and gtk apps (but first opera) to use native transparency.

I'm using the Oxygen Transparent Style. The point is that every native KDE app has transparency in both titlebar and windget background, but in Opera 11.10, which is QT if I'm not mistaken, they are both opaque. Playing with the "specific window overrides" i've managed to get the transition between the "tab" of the window decoration and the browser tab bar smoother (by applying to Opera the forze solid background option), but now I want a way to make that grey color into native transparency, like the one in the KMail shot. And, if possible, apply this to every GTK app (which basically applies to GIMP, GParted and Liferea).

For opera integration, you should check this thread:
viewtopic.php?f=14&t=25420&p=186472

As for transparency on GTK apps, I don't think this is currently possible. The following are the closest things I've seen for unifying the look-and-feel across QT and GTK apps: The Oxygen GTK theme - imitates the Oxygen Qt theme for GTK apps, but I'm not sure if it carries over the transparency settings. The QtCurve theme - transparency works for some GTK apps but crashes others. The gtk-qt theme engine - attempts to use Qt for rendering the GTK widgets; my experience is that the rendering tends to be poor and transparency does not work. The QGtkStyle engine - goes the other way, using the gtk style for Qt apps (I haven't tried this one. I imagine transparency depends on finding a gtk engine which supports it).

Opera is not a full native Qt application. It uses Qt in some places and it's own rendering capabilities in others.

I read somewhere opera used X11 native rendering instead of just QT, so no surprise. I know it's plain hard to make GTK apps to look transparent without crashing: what I now want to know (after confirming what you said) is if I can apply the Blur and Transparency to every pixel of the grey color that gets opaque when transparency isn't supported.

BTW: if you watch carefully the shots, i'm actually using the opera intefration tips (oxygen native theme) and the oxygen-gtk theme, so it looks at at least partly integrated.

BTW2: Why doen't KWin get the edecoration transparent when the widgets can't be so?