How to enable a:focus customisation for theme?

[xnotyuki]

I want to customise a:focus for the theme so that when I press tab, the selected link will have a highlight. Presently, there's no a:focus customisation on Next at all, but I don't know how to make this edit. Would appreciate any help.

[stevenjoezhang]

You can use a custom CSS file to set a:focus. Ref: https://theme-next.js.org/docs/advanced-settings/custom-files#Hide-Keep-on-posting-in-Archive-Page
It's similar to Hide "Keep on posting" in Archive Page in the docs, for example

a:focus {
  color: red;
}