The following style params are available: 

body.background:  
body.margin: 0px
table.font-size: normal
table.font-weight: normal
table.width:  
table.height:  
a.color: black
a.font-weight: normal
a.text-decoration: underline
table.border-spacing: 0px
table.border-collapse: collapse
table.border: 0px solid black
table.background:  
td.padding: 0px
td.padding-right: 4px
td.background:  
td.author.color:  
td.subject.color:  
td.date.color:  
td.author.background:  
td.subject.background:  
td.date.background:  

In addition, the following general params can be specified:

group (the group name)
articles (the number of lines)
type ('topics' or 'articles')
content (on the form 'subject,from,date')

Example URL to use in your iframe:

http://blog.gmane.org/recent.php?group=gmane.linux.kernel&type=topics&td.author.background=green&td.author.color=white&table.border=1px+solid+black