Finally, and thanks to Xubuntu guys, the notifications in the Lubuntu desktop will look properly. This is an old bug (see #1362555), affecting multiple themes, crossing and assigning incorrect values to some GTK widgets. But now they’re fixed. If you want to fix it in your system, you must add the Lubuntu Artwork PPA to your system (instructions here). This is how it looks now:
Remember these ugly notification messages?