Improve styles for accessibility

[coycatz]

Description of proposed changes

The goal of this pull request is to improve the accessibility and readability of the Auspice user interface by increasing text contrast and font weight in the footer and filter badges.

Changes:

• In src/components/framework/footer.js, the default text color has been darkened from rgb(136, 136, 136) to rgb(3, 3, 3) to provide better contrast against the background.
• Throughout the footer (including paragraphs, lists, and tables) and in the filter badges (src/components/info/filterBadge.js), the font-weight has been increased from 300 to 400 to enhance legibility.

Related issue(s)

nextstrain/nextstrain.org#1054

Checklist

• Checks pass
• If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR
• (to be done by a Nextstrain team member) Create preview PRs on downstream repositories.

[victorlin]

@coycatz thanks for the PR! I've fixed the formatting of the description and updated the link to the original issue. A few notes:

• I've removed the Fixes GitHub keyword from the description because the PR is only a partial fix for the linked issue – there are still other styles to be addressed before fully closing.
• These are user-facing changes which deserve a changelog entry.
• I added the "preview on nextstrain.org" label but forgot that it doesn't work for fork PRs. I'll manually create a preview PR and link it back here.