feat(design): add default theme and update docs

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

There is no default theme set in @daffodil/design.

Fixes: N/A

What is the new behavior?

Set a predefined theme in @daffodil/design so that users do not have to configure a theme if they choose to use Daffodil's default theme.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information