Ok, finally solve with the help of Gtk Inspector. With these rules you can customize a dark theme for sidebar and dialogs (I didn't want to customize entire interface):
#GeanyDialog GtkButton, #GeanyDialog GtkButton GtkLabel, #GeanyDialog GtkButton GtkImage, #GeanyDialog GtkWidget GtkEntry, #GeanyDialog GtkWidget GtkButton { background: #f0f0f0; color: #000; }
#GeanyDialogSearch, #GeanyDialog, GtkTreeView { background: #1e1e1e; color: #fff; }
#GeanyDialog GtkWidget { background: transparent; }
GtkTreeView:hover, GtkTreeView:insensitive, GtkTreeView:active { background: #666; }
GtkTreeView:selected { background-color: #2F5692; color: #fff; }