Revise black focus outlines in welcome experience

[rossmoody]

Need to revise the focus styling in the welcome experience away from the thick black squares.

[bsclifton]

Related / dupe of #3633

[kjozwiak]

@rossmoody can you please add a screenshot/example of how the new focus lines should appear under brave://welcome. This way QA will be able to tell if what we're seeing is actually the expected/intended behaviour 👍

Are the focus lines changes anywhere else, or this strictly for the brave://welcome page?

[btlechowski]

Verification passed on

Brave	0.68.128 Chromium: 76.0.3809.100 (Official Build) beta (64-bit)
Revision	ed9d447d30203dc5069e540f05079e493fc1c132-refs/branch-heads/3809@{#990}
OS	Ubuntu 18.04 LTS

The ugly focus outline is gone. Checked all welcome sub-pages.