Yeah, if you had the full KDE installation, you'd be able to switch the KDE theme, and by extension, change how Okular looks. Okular is a KDE app, so it would look more at home in a KDE desktop, rather than XFCE.
I'm personally not a fan of mixing QT with other GUI toolkits like GTK/Tk/whatever, because they generally don't play that well with one another. I'd recommend trying a different PDF viewer, like xPDF... For me personally, that kind of broken rendering is a showstopper, even if I like the features otherwise.