After successfully setting up and configuring svnspam, I noticed that the emails sent to my Gmail account are missing the colored diffs. When examining the original email using Gmail's view original feature, I found the CSS code as follows:
<html>
<head>
<style type="text/css">
body {background-color:#ffffff;}
.file {border:1px solid #eeeeee;margin-top:1em;margin-bottom:1em;}
.pathname {font-family:monospace; float:right;}
.fileheader {margin-bottom:.5em;}
.diff {margin:0;}
.tasklist {padding:4px;border:1px dashed #000000;margin-top:1em;}
.tasklist ul {margin-top:0;margin-bottom:0;}
tr.alt {background-color:#eeeeee}
#added {background-color:#ddffdd;}
#addedchars {background-color:#99ff99;font-weight:bolder;}
tr.alt #added {background-color:#ccf7cc;}
#removed {background-color:#ffdddd;}
#removedchars {background-color:#ff9999;font-weight:bolder;}
tr.alt #removed {background-color:#f7cccc;}
#info {color:#888888;}
#context {background-color:#eeeeee;}
td {padding-left:.3em;padding-right:.3em;}
tr.head {border-bottom-width:1px;border-bottom-style:solid;}
tr.head td {padding:0;padding-top:.2em;}
.task {background-color:#ffff00;}
.comment {padding:4px;border:1px dashed #000000;background-color:#ffffdd}
.error {color:red;}
hr {border-width:0px;height:2px;background:black;}
</style>
</head>
<body>
<table cellspacing="0" cellpadding="0" border="0" rules="cols">
Interestingly, when I copy the entire HTML content from the original source and save it as an HTML file locally, the colored diffs display perfectly. This makes me wonder what could be causing the issue with the emails.