Q: how to specify custom header shading style?

I'm probably missing something obvious (or it may be just jetlag from travelling;) but I can't quite make a custom header work.  Please note, I am no expert on CSS, so my understanding of how to do this could be completely wrong.

Here is the standard type of table header:

[cols="^1,4", options="header"]
|===
|col1header|col2header
etc

When generated, this connects 'header' to the following:

<th class="tableblock halign-center valign-top" colspan="3">...</th>

i.e. it's picking the th.tableblock CSS class.

Now, what I want is a custom table header shading, just for some tables, e.g. on the front page. If I defined a new class like (is this the right thing to add to the .scss file?):

th.tableblock.frontpage {
   background-color: lightsteelblue;
}

what do I put in to the Asciidoc header line to generate the right HTML - something like the following I think:

<th class="tableblock.frontpage halign-center valign-top" colspan="3">...</th>


Any suggestions appreciated.