Improve styling for the Traits tutorial

[mdickinson]

The CSS used for styling the appearance of the Traits tutorial in the etsdemo app could use some work. See discussion in #1418 and #1419.

[mdickinson]

Re-opening: #1421 improved the situation, but there's still work to do.

[corranwebster]

For reference, specifically we are seeing things like this:

Note the slightly larger first line in the code section.

[mdickinson]

I'm also seeing non-fixed-width fonts in code samples; see the #1421 discussion for details.