Add dark theme colors to pullquote and quote

[j-gosch]

Description

Closes #16732

This pull request adds dark theme colors to pullquote and quote blocks to address visibility issues on themes that enable dark-editor-style. I changed the colors of the borders to ones suggested in this comment by @kjellr. I also noticed that the citation in both blocks were using dark gray and changed both to be a light gray instead.

How has this been tested?

I enabled the TwentyTwenty theme and changed the background colors to enable the darkmode. I then made the code changes, verified the changes and then changed the background back to a lighter color to ensure no harm was done to the default light theme.

Screenshots

Before

After

Types of changes

Feature: Darkmode for quote and pull quote blocks

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
  Follow-up issue Check color contrast and provide recommendations to meet WCAG guidelines #16826 for general color contrast
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[t-hamano]

Hi @j-gosch,

I apologize for being so late in confirming this PR.

This issue appears to have been resolved by #26510, so I'd like to close this PR, but I appreciate your contributions 👍