how to make footer links open in new tab?

you can do this by adding target="_blank" to link, for example:
<a href="mylink.com" target="_blank">Link</a>

it doesn’t work in the widget as it only allows me to type the website, can I do something in CSS?

You would need to add this to specific code from your template.

how do you mean?

For starters you need to give name of your template.

sorry https://helenaclayton.sennett.co.uk/

its central by theme forest

For matters concerns commercial theme you need to contact author directly.

You can look for some kind of footer widget php file with href variables, but i can’t really give you any specific info in that matter.