One would assume that if the CMS is homegrown that whatever creates the list can add class="odd" or class="even" to those tags.
And the subject might be a misnomer, because it wouldn't generate zebra stripes unless the indents were strictly alternating, like this:
- light
- dark
- light
- dark
or like this
- light
- dark
- light
- dark
If it was a straight list, and a greenbar-like striping had been intended, and if you did not have control over the HTML generation, you could use CSS2 with an "adjacent sibling" + selector, such as
li.topic, li.topic + li.topic + li.topic { background: #546A94; }
li.topic + li.topic, li.topic + li.topic + li.topic + li.topic { background: #558; }
CSS specificity rules should ensure that the line with the most matching li.topic in it (i.e the one that fits best) is used for display. The advantage over a scripted solution is that it works even when scriptingis turned off in the browser (but would the CMS work at all then?).