Box is the default Lubuntu theme, and it should cover all the system needs (icons, controls, backgrounds…) but some applications don’t follow the XDG specs for their buttons and graphics. FileZilla is a must-have app for most desktops, so here’s a theme for it. To use it go to Artwork, grab it and extract the folder to /usr/share/filezilla/resources/ folder. Then open FileZilla, at the menu Edit / Preferences and look for themes. That’s all!