This is actually in the next XenBase update but for now you can add this into your extra.

.messageList .message:nth-child(2n) {
background-color: #FFFFFF;

I tried with a darker shade than the current grey and it didn't look all that great, the white/grey looks good(personal opinion).
With the advertising mod you have setup not sure if theres an easy fix, suppose a selector for just first post are the ads showing on all posts for everyone

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