Update theme's look for GNOME 3.12+

[garrett]

It's really hard to think how to reskin Thunderbird to fit in with modern GNOMEy goodness without touching the UI.

There are tabs, but they don't switch documents; they actually switch complete UI contexts. There are little buttons to the side of the tabs. The toolbars are under the tabs (as the tabs switch what they do). The main menu is on the contextual toolbar. It's basically a mess.

Calendaring: There's sometimes a sidepane showing calendar events and tasks (if one uses the popular Lightning extension).

Conversations: Many people are also using conversation view too, so we'd want to take that into account as well.

I really don't know what is most proper for the app.