WHCM styles improvements

Based on the audit from @shyusu4, see Wagtail.org | Accessibility audit spreadsheet. WHCM fixes:

Issues

[x] Close icon in the search modal isn’t visible.
[x] Menu toggle isn't visible.
[x] Copy icon in "pip install Wagtail" button disappears when clicking on it.
[x] (not in audit) SVG icons in buttons now match the button text color
[x] Social media icons are not using the correct color (linktext)
[x] All social media, “sponsor wagtail”, and “contribute code” icons aren't visible.
[ ] #463
[x] Arrow icon within CTA button isn't visible when hovered.
[x] Arrow icon within CTA link in sub-navigation panel isn’t visible.
[x] (Blog posts) Code blocks should have a clear separation from regular content.

Assigning this to @shyusu4 as part of her Outreachy project