fix(vibe-check): make logo readable in dark mode

[DerGeraetK]

Changes

Fixes #17952

In dark mode on /vibe-check, the PostHog logo in the "You'll hate PostHog if..." heading was nearly invisible: the Logo SVG letters use hardcoded black path fills that don't adapt to the theme.

This applies the same theme-conditional fill already used for the Logo across the site (About/v2/Letterhead.tsx, Home/Control, Home/Test, ko/_KoreanHome.tsx):

fill={siteSettings.theme === 'dark' ? 'white' : undefined}

• In dark mode the whole logo (letters + glyph) renders monochrome white — intentional, matching the site's established dark-mode logo treatment (see screenshots).
• In light mode undefined falls back to the component default — unchanged behavior.

Note: open PR #17950 fixes the text contrast on this same page (#17926). This PR only touches the Logo fill — semantically independent, but both edit adjacent lines of the same <h1>, so whichever lands second needs a trivial rebase. Happy to rebase if #17950 merges first.

Screenshots (dark before → dark after → light unchanged)

Dark before:

Dark after:

Light unchanged:

Checklist

• I've read the docs/content style guides
• Words are spelled using American English
• Relative URLs for internal links (n/a — none added)
• Vercel preview build checked (will verify once the preview builds)
• No page moved (n/a)

🤖 Agent context

Claude-written: implemented with Claude Code under my direction.

• Tools: Claude Code (single session). Approach: located the root cause in components/Logo (hardcoded path fills), searched the repo for existing dark-mode Logo treatments and applied the established pattern rather than inventing one.
• Verification: ran the site locally (Gatsby dev), visually verified both themes on /vibe-check; Prettier/ESLint via the repo's commit hooks.
• Human review: I reviewed the diff and can explain it.