Oliver, another Ars Technica reader, has taken Mithras’ MailWideScreen bundle and tweaked it some more, following the advice of Malacoda.
He added alternating background colours for the rows in Message Viewer, vertical lines and small scrollbars.
He has posted screenshots of the result in the Ars Technica thread 
.
These tweaks are done by editing the nib files in Mail.app’s Resources Package with an app called Interface Builder, which is part of the Mac OS X Developer Tools.
These tools are not installed by default, but you can find them on your installation discs, where they live in a folder called “Xcode Tools”. After installation you will find them in a folder called “Developer” in the root directory (this may not be the right word, but a hangover from pre-Mac days) of your harddrive.
If you have them installed, you can navigate to the MessageViewerContents.nib file in your Contents/Resources/English.lproj/ folder of Mail.app’s package, click on it and it will open in Interface Builder.
At this point, I should confess that I have absolutely no idea what I am doing in Interface Builder, so if you want to read on and fool around with the way Mail looks (as I did), it’s at your own risk.
I recommend backing up Mail.app at the very least, or creating a duplicate of Mail.app and dragging it to the Desktop for safe keeping. Closing Mail first seems like a prudent step too.
It’s easy enough to create Oliver’s extra tweaks. When the nib file opens, click once on the Column view of the main window to select it:
Then bring up the Inspector, pictured above on the left, by pressing Shift-Command-I or Window > Show Inspector. All the options are there. Save your changes, quit and you’re done.
But once you start, it’s hard to stop. There’s lots more MailWideScreen-independent fun to be had. You can slim down the scrollbars whether you use the hack or not:
Or you can give the message viewer a once-over all your own. Here it is with alternating colours and horizontal lines:
I decided to stop before I busted something. But you don’t have to.
UPDATE: The MailWideScreen plugin has a new name: Letterbox
Slick, thanks. I’ll roll these options into the plugin when I get a chance.
I was hoping that you wouldn’t be offended by people mucking around with your excellent work like this.
Especially people with no idea what they are doing :)
One thing I would add here is to use a degree of caution — some things are easy to modify in the nibs and some things are not so easy and cause some real difficulty that may not show up immediately — certainly hacks such as switching up the scroller for a mini scroller is probably safe, but reorganizing things can lead in to significant headaches that interfere with Mail (or its plugins.) Also definitely don’t muck about with connections (and don’t think you can copy and paste things)
It cannot be overstated — if you hack the nib, back things up!
Also note that any update that Apple does for Mail will likely wipe out your hack.
Definitely using plugins to effect the alterations are the better way to approach things, as they effect changes at a code level and can more significantly integrate into Mail and how mail is coded. Also alterations effected by plug-ins are not going to be undone with updates of Mail. (also the alterations given by plugins are way more cool :)
(Brought to you by a friendly plugin developer :)
hehehehehe…. :)
I tried the June 1 version for 5 days on my 15″ PB, tweaking the columns to maximize what I could see. I finally removed the bundle because I got tired of having to scroll right on emails that were too wide. So, on the trade-off between having to scroll horizontally or vertically, I’m sticking with vertically. At least for now.
This just proves how flexible a real Cocoa app can be. We can make all these changes without having access to source code!? Sweet!!!
[...] bei Hawk Wings gibts ein paar Tricks, um mit Hilfe des Interface-Builders das aussehen von Mail etwas Widescreen-freundlicher zu gestalten, zB. mit schmaleren Scrollbars. [...]
[...] Using the tips for hacking the Letterbox plugin, you can start to make changes to the nib files which control the layout of Mail.app’s message list and preview pane. [...]
Can anyone comment about how I can follow this thread to change the mail background color? 3 pane view, over under with mailboxes to left side, text on bottom. I did all written above but I can’t see how to get a different background color?
I want a black background with white text if I can. I have Mailsmith that way and I had Eudora that way when I used it. I hate staring at the white screen glare all day long, stresses my eyes.
Bonus points if anyone can extend this into the mailbox window and also the mail list similar to Gyaz Mail.
Thanks.