Tweaks the 6.4 about pages in a couple of ways:
- Decouples the background from the 6.4 logo so the logo can move for RTL.
- Updates a color to improve color contrast.
- Help prevent overlap of long text strings with 6.4 logo.
- Ensure background isn't dark when no background is used on mobile.
Props nudge, jorbin, afercia, sumitsingh, sabernhardt.
See #59289, #59664.
Built from https://develop.svn.wordpress.org/trunk@57018
git-svn-id: http://core.svn.wordpress.org/trunk@56529 1a063a9b-81f0-0310-95a4-ce76da25c4cd
