These are loaded from a CSS file that is included in your login.html, index.html or index3.html. You can edit this (found in your theme directory). To preload these files as suggested.
Thanks for the suggestion, but my index.html seems to use some sort of templating to load the CSS:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<title>Simcenter Webapp</title>
<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1, maximum-scale=1">
<meta name="apple-mobile-web-app-capable" content="yes">
{{themecss}}
I presume {{themecss}} gets swapped out by some Java templating system…so it appears this is controlled by mendix under the hood….