It's not a huge problem by any means, but I'm curious about why it happens.

I'm running openbox, KDE stuff with the plastique theme, and via gtk-qt-engine (I guess), most of the gtk stuff follows the same theme, as set by lxappearance. Except for epiphany and zenity (perhaps something more, but nothing else I've tested from the list resulting from "apropos gtk").

I've even tried running both with commands like "GTK2_RC_FILES=... zenity" and "export GTK@ [...]", but doesn't work with these two, but it works with pcmanfm (on which I tested a different theme from the one I'm using, of course).

Epiphany is a Gnome application, so maybe is because of this that it is weird under KDE. However, applying the theme again should solve your problem I guess. Maybe choose another theme, applying, and the going back to your favourite theme would do it.

It's not exactly under KDE, but openbox. It's weird that some specific GTK applications have this issue and others don't, though.

And things just got weirder with some recent upgrades (I guess). Evince joined the club of not-following the qtcurve theme.

I've noticed that if I ran gnome-settings-daemon, it will takeover and change the theme of even those GTK apps that are following qtcurve. I guess that there's some other variable than GTK2_RC_FILES that is relevant to this whole mess.

Unfortunately, along with the recent changes, apparently the default gnome theme changer disappeared, along with some other stuff on gnome-control-center. I guess I'll try to install more stuff from gnome environment/session just to see if I can get the settings right. Right now the actual gnome session won't even work (without giving any useful information on why not, just a sad face on a computer icon and asking me to log out and try again), only a "fallback" gnome session does.

You need to choose a theme that supports gtk3, otherwise as they update these gnome apps will stop being displayed correctly.

I was trying to fix it again for a while, and it seems that GTK3 stuff are only theme-able under Gnome 3. Running under another DE (well, I just tested under openbox) they have that windows 3-11ish naked look, with aliased fonts. A different session, a gnome-session running at the same time, will have a theme and the fonts will look nice.

I'll see if there's something, hopefully something tiny that can be run under a openbox session to have the theming or if I can downgrade some apps to gtk2. I wish it had back-compatibility with gtk2...

Better yet, I wish I had everything I needed in qt/kde apps, and that those were as stable as the ones made with gtk... it seems that one side has usability and nice looks, and the other has stability in exchange of usability and some looks. Or total compromise of looks if you don't adhere to the whole package.