Fix inconsistent theme-color values

[niinaad5]

What does this PR do?

Fixes inconsistent theme-color values across the website.

The site-wide theme color was still using the old blue value, while several pages also had legacy custom colors that no longer matched the current white navigation/header.

Changes

• Change the site-wide theme-color from #3689e6 to #fff
• Remove the outdated theme-color override from the Get Involved page
• Remove the outdated theme-color override from the Thank You page
• Remove the outdated theme-color override from the documentation page

The existing theme-color handling in header.php remains unchanged.

Testing

• npx gulp

• PHP syntax checks for all modified files

• Stylelint

• Verified rendered theme-color on:

  • Homepage
  • Get Involved
  • Documentation
  • Thank You

All tested pages now render:

<meta name="theme-color" content="#fff">

Fixes #3146