UHF-8673: Fix Metro button contrast

[xkhaven]

UHF-8673

What was done

• Metro color theme had wrong contrast on buttons

How to install

• Make sure your instance is up and running on latest dev branch.
  • git pull origin dev
  • make fresh
• Update the HDBT theme
  • composer require drupal/hdbt:dev-UHF-8673_Fix_Metro_button_contrast
• Run make drush-cr

How to test

• Banner with a link using style of "Full colour background" has black background and white text, hover has transparent background and black text
• Banner with a link using style of "Only borders" has transparent background, black borders and black text, hover has black background and white text
• Check that code follows our standards

Designers review

• This PR does not need designers review

[teroelonen]

This is good 🦖