Style.Add on table cell doesn't work

I have some code inside a loop and I want to change background on some of the cells and here is the simplified code of what I'm trying to achieve (in WebForm.aspx.cs)
test.Style.Add("background-color", "red"); sb.Append("<tr>"); sb.Append("<td class='test' id='test'>"); sb.Append(myContent1); sb.Append("</td>"); sb.Append("<td>"); sb.Append(myContent2); sb.Append("</td>"); sb.Append("</tr>"); Something seems to be wrong with "test" because when hovering over the name I get: "The name 'test' does not exist in the current context". On the other hand if I create something with the same ID on WebForm.aspx then it works but I need to have this in the code behind file because the TDs are dynamically generated inside a loop.
Which cells a ...