R1:892f527b9f63

Improve remarkup codeblock syntax highlighting when using Dark Mode

Summary:
PhabricatorDefaultSyntaxStyle works now with 'CSS-color-aliases' instead of fixed colors.

The aliases are translated in the corresponding themes CelerityPostprocessor.

| Before | After |
|-----------|-----------|
| {F709419} | {F709421} |

Ref T15056

Test Plan:
1) bin/celerity syntax
2) bin/celerity map
3) bin/cache purge --all
4) sign in
5) go to user's Settings > Display Preferences and select the Accessibility (user…