I think I managed to replace (nearly) all blues with roughly corresponding gray values, though I needed to change global.css as well to make the 'New Topic' etc. buttons gray.
Disclaimer: it looks worse.
To test, save a copy of any attnam page, replace
<link rel="stylesheet" href="/css/global.css?v=2">
<link rel="stylesheet" href="/themes/blue/silver_023.css?v=2">
by
<link rel="stylesheet" href="global.css?v=2">
<link rel="stylesheet" href="style.css?v=2">
and paste
style.css and
global.css in the same folder.