If one is going to write pompous posts one should be paying more attention to the details. As Kev Quirk kindly pointed out the colours were all screwed up on a small mobile screen and suggested some CSS to correct them…

It seems the SEMPress theme is inheriting the colour setting from the title when viewed on a small mobile screen – the mobile version on an iPad was showing correctly.

The WordPress theme customiser allows you to add CSS to override the original settings. So I left the text dark, set the title’s yellow colour and removed the wayward shadowing with the added CSS:

#site-title a {
          color: #ff0;
}

#site-description {
   font-size: 2em;
         color: #ff0;
}

body {
   text-shadow: none;
}

Which seems to have done the job but has probably screwed up something else somewhere.