Text in footer (Copyright area) won't centre

This problem only appears on my pc in full screen mode, smaller screen sizes display correctly. I’m using Windows 10 and a recently updated version of Firefox, but the same problem occurs in Opera, Safari and Chrome. The website is
https://www.blueheapmedia.co.uk

I’ve Entered the content I want into the Footer – Copyright Text box in the customizer and it appeared left aligned in the copyright bar on the page in full screen mode.

I added

.copyright {
text-align: center;
}

into the custom css area which centred the text in what looks like the left hand half of the copyright bar.

Any thoughts on how to centre the text properly?