Bootstrap custom theme bgcolor not applied to fixed header

PeteDavis

I have a table with a fixed header. All works OK when the default bootstrap stylesheet is applied. When I add a theme stylesheet, darkly.css, the header bgcolor changes from dark grey to white when the fixed header starts to float.

Example at http://race-results.info/test2017/20170222mlsu/index3.php

I'm not a css expert and I'm struggling to understand if this is an issue with darkly.css or with one of the datatables css files. I'm calling these stylesheets:

<link rel="stylesheet" type="text/css" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/fixedheader/3.1.2/css/fixedHeader.dataTables.min.css">
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/1.10.13/css/dataTables.bootstrap.min.css">
<link rel="stylesheet" type="text/css" href="../../lib/stylesheets/darkly.css" />

The Darkly theme is here:

https://bootswatch.com/darkly/bootstrap.css