The chatter's default color stripes have now been removed from the Red and default (Blue) themes (they were never default in any other themes).
You can add colored stripes back in to your chatterbox by going to Display Settings and adding the following to your personal CSS:
where "#FEE" specifies a soft pink color. The first "td.highlight" controls the 'even' rows in non-nodelet chatter such as fullpage chat while the "tbody.nodelet td.highlight" controls them in the Chatterbox nodelet.
If you want the 'odd' rows to be a non-default color, then you can use
Note that specifying the color for the 'odd' rows in that way meant that the selectors for the 'even' ("highlight") rows had to become more specific.
I'm sure there are also other ways to do these. And feel free to post suggestions on how to customize the spacing, punctuation, etc. (you can all 'view source' to see what CSS classes and such are available for selecting on).
(I'd personally rather have the post-user-name "]" and ":" enclosed in a single 'span' instead of each having their own, but not enough to champion the effort / thrash to change that.)
Update: My prior update claiming that I'd made an mistake was in error.