I use a custom Gtk theme along with the default Ambiance window controls. Using my theme, the Unity panel shows its very ugly default buttons when my window is maximized. Since I have the same window theme, I am assuming this comes from the Gtk theme. I tried copying and pasting a few things from Ambiance's gtkrc, but nothing has stuck so far.
What must a theme author do to gain pretty looking window buttons in Unity's panel?
Dylan, metacity theme support has not been implemented yet in Unity. It's not a priority but someone is working on it ATM. I hope to see it in next unity release (sometime this week).
here is the bug report: https://bugs.launchpad.net/unity/+bug/740232
Owais is right but there is a work around as long as the theme you want to use matches the default Ambiance window borders.
Backup Ambiance:
Replace Ambiance:
Logout and log back in. As an extra bonus, by "high jacking" the Ambiance theme the login screen is also themed. I have used this with the darker Equinox themes and have had no issue.