It's quite likely that that's done with CSS, but you could hard code it in the files.
On default.asp you'd have to change the bgcolor attribute in any of the following lines:
500 (in red):
Response.Write " <tr>" & vbNewline & _
" <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top"">"
if ForumFType = 0 then
513 (in red):
end if
Response.Write " bgcolor=""" & strForumCellColor & """ valign=""top"">" & _
"<font face=""" & strDefaultFontFace & """ size=""" & strDefaultFontSize & """><span class=""spnMessageText""><a href="""
528, 530, 533 and 535 (in red):
if IsNull(ForumTopics) then
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top""><font face=""" & strDefaultFontFace & """ color=""" & strForumFontColor & """ size=""" & strDefaultFontSize & """>0</font></td>" & vbNewline
else
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top""><font face=""" & strDefaultFontFace & """ color=""" & strForumFontColor & """ size=""" & strDefaultFontSize & """>" & ForumTopics & "</font></td>" & vbNewline
end if
if IsNull(ForumCount) then
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top""><font face=""" & strDefaultFontFace & """ color=""" & strForumFontColor & """ size=""" & strDefaultFontSize & """>0</font></td>" & vbNewline
else
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top""><font face=""" & strDefaultFontFace & """ color=""" & strForumFontColor & """ size=""" & strDefaultFontSize & """>" & ForumCount & "</font></td>" & vbNewline
end if
551 and 553 (in red):
if (strShowModerators = "1") or (mlev = 4 or mlev = 3) then
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""left"" valign=""top""><font face=""" & strDefaultFontFace & """ color=""" & strForumFontColor & """ size=""" & strFooterFontSize & """><span class=""spnMessageText"">" & listForumModerators(ForumID) & "</span></font></td>" & vbNewline
end if
Response.Write " <td bgcolor=""" & strForumCellColor & """ align=""center"" valign=""top"" nowrap>"
if ModerateAllowed = "Y" or (lcase(strNoCookies) = "1") then
Changing the bgcolor attribute on the alternating cells ought to do the trick on default.asp
Just keep in mind that 528 and 530 are the same cell, just a different value will be entered, and the same goes for 533 and 535.
I did not test this and therefore cannot be 100% sure that this will work. Make a backup of your files before you edit them!