Is there a way to globally change the appearance of messages in Messages (Beta) for OS X? In OS X Messages (Beta), the appearance of a given message thread can be changed by selecting submenu options from the View &gt; Messages menu item; but this only applies to the currently displayed message. Is there a way to configure these options as defaults for (all) new threads?
Yes, there is:

Messages &gt; Preferences &gt; Messages


You can set the look and feel for incoming and outgoing messages here.