What does this PR do?

Supports the theme's focus.shadow to accept a string.

Where should the reviewer start?

theme file with types

What testing has been done on this PR?

local tests.

How should this be manually tested?

locally.

Do Jest tests follow these best practices?

N/A

Any background context you want to provide?

On src/js/utils/styles.js we use:

return `
      outline: none;
      box-shadow: ${focus.shadow};
    `;

and since box-shadow accepts the type of string, it should be accepted as a valid input type as well.

What are the relevant issues?

#5446

Screenshots (if appropriate)

Do the grommet docs need to be updated?

Where are the global theme types are documented? the new type should be added there.

Should this PR be mentioned in the release notes?

yep, as a TS fix.

Is this change backward compatible or is it a breaking change?

backward compatible