TP-1263 fix faulty multibutton theming

[IiroJarv]

Actions necessary for applying the changes: (for example composer install; drush updb; drush cim)

Drush deploy

Testing instructions:

• Login and go to search page
• Check that:
• Filters have blue background when selected
• Dropdown has high enough z-index
• Filters don't have double border
• Filters blue background doesn't "leak" from under it
• Go to group page and confirm same things.

Review checklist:

• The code conforms to Drupal coding standards
• I have reviewed the code for security and quality issues
• I have tested the code with the proper user roles
• I have moved the ticket to the approval lane, added testing instructions and assigned it to the product owner.