If you want to change the default font, layout, colours and the signature of your Messages, then this can be achieved like so:
Click on the top right tools spanner or any "Tools" main menu.
Then choose [Admin \ Edit Message Defaults].
Now choose which default you want to change. I would start with "New Message Rich Text". You can then copy/paste it to "Reply to Rich Text" and "Forward to Rich Text". You are free to have all of them open at the same time to aid your design.
When the editor opens, simply style it to your desire. Remember this is Email so don't go crazy with large images. Also remember to leave a blank line at the top as this is where your edit cursor will go when this template is used.
After you're happy with your creation, [Save] it then try sending a test Email to see if it looks okay.