Update logo and font sizes

[alexandratran]

Add darkmode-compatible wordmark to navbar and footer, and adjust navbar title and header font sizes.

fixes #74 fixes #75

Preview: https://metamask.github.io/mm-docs-v2/preview/4d0fdf95b4cdcb736eb46c7ce19e15af18fb449a/wallet/

Previous:

New:

[alexandratran]

@metamaskbot publish-preview

[github-actions[bot]]

Preview published: preview/4d0fdf95b4cdcb736eb46c7ce19e15af18fb449a