Allow navigation via tab key in welcome page

[cezaraugusto]

fix brave/brave-browser#5504

Welcome page navigation via tab is impossible without this change since all non-visible content is hidden visually but still accessible via keyboard. This change fixes it and improves other element focus.

Test Plan:

1. Open welcome screen
2. Navigate via tab

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[bsclifton]

Keyboard navigation works great! However, having a hard time seeing the outline in dark mode.

Light mode seems fairly easy to see the orange outline, but here's a clip of tabbing around in dark mode:

• outlines for the bullets are barely visible
• Skip welcome tour does not show any kind of focus (this is the last thing I navigate to and press space, which launches into a new tab)

[cezaraugusto]

@bsclifton thanks for reviewing, updated

[petemill]

This seems like something that would be useful for all brave components. Is there a reason outline color would be something that welcome page would have only?

[cezaraugusto]

I agree it would be useful for all brave features but adding this global default here seems outside of this PR's scope. I'm happy to do that in a follow-up task where we move our brave-core themes to this repo

[petemill]

I don't think it's outside of this PR scope since this is the first PR that's adding this value. Either way requires a new brave-ui version - allowing for separate layer of themes (which you've discovered is problematic and requires using the drastic any step, thereby removing all type checking for theme props), or simply adding this new property.

[bsclifton]

MUCH better - looking good! 😄