Why is it that Videos' color scheme is dark grey when the other apps seem to be light grey? It kinda stands out and looks odd as a whole. The rest of the apps appear to go together.
To expand on Peter's answer, many GTK+ themes (elementary included) have a dark variant which can be enabled for any GTK+ application by setting
Gtk.Settings.get_default ().gtk_application_prefer_dark_theme = true;. You can find the corresponding line which the Videos app uses to toggle the dark theme here if you're interested.
As to why, it emphasises the actual content and minimises distractions from light pollution. The Gnome developer documentation has this to say:
Some applications benefit from minimizing the amount of light pollution that interferes with the content. Good candidates for dark themes are photo and video editors that make the actual content get all the attention and minimize the distraction of the chrome. Dark themes should not be used for documents, where large spaces are white/light and the dark chrome creates too much contrast (web browser, text editor...).