Editor: Remove some hardcoded text color values from patterns that have body...

Editor: Remove some hardcoded text color values from patterns that have body text with no specific background color.

This ensures that the patterns respect the theme's default color for body text and avoids contrast issues, particularly in dark themes.

Props nrqsnchz, mapk, fullofcaffeine.
Merges [48864] to the 5.5 branch.
Fixes #50892.
Built from https://develop.svn.wordpress.org/branches/5.5@48865


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48627 1a063a9b-81f0-0310-95a4-ce76da25c4cd