It was tested on another computer where the website was opened for the first time..
I have two css (style.css and editor-style.css) but all the “border-radius” are in the main style.css like you wrote .
Weird is, that when I check the site in some online checking service like browserstack.com it seems to be ok.
I will try to test the site on more pc´s and we´ll see..
I’m fixing the yellow border issues alongside each post (thanks for the screenshot!) in the next theme release, but the border-radius property isn’t supported by IE8, which is why you see squares instead of circles there.
As Andrew pointed out, the only “fix” is to use a script or plugin that allows IE 8 to display modern CSS properties. I haven’t used these plugins myself so can’t vouch for how they work, but you might ask the plugin developer for assistance.
(Beyond that, I’d recommend upgrading your browser to IE 9, or using Chrome or Firefox instead of IE — http://browsehappy.com/)