This is how the GtkInfoBar we have used to work, but at some point the gnome-themes developers decided that they didn't like that anymore. See https://bugzilla.gnome.org/show_bug.cgi?id=704266. The white-on-blue bar is kind of terrible to read, anaconda uses the warning and error colors to attract attention to warnings and errors, so this style change undoes the color changes and makes the infobar colors jarring again.
ACK.
- Chris