Problem with altered F2 theme not displaying properly on some PCs

I’ve used the F2 theme as a basis for my site, https://www.ourchoicecoventry.org.uk, and it looks fine on my Mac, a Mac Book Pro and my Acer laptop. I’ve also seen it on some other PCs and it looks as it should. However, I know that on some older computers, it doesn’t display properly: there are no margins on the content which buts right up to the right sidebar; the content of the right sidebar drops to below the main content text, and the menu items wrap round and revert to the original F2 colours.

I think what’s happening is that some of the original F2 settings are overriding the new Our Choice style sheet. Can anyone tell me how to resolve this?