Fix email buttons for white theme

apps/theming/lib/Capabilities.php

apps/theming/lib/ThemingDefaults.php

apps/theming/tests/CapabilitiesTest.php

lib/private/Mail/EMailTemplate.php

lib/private/legacy/defaults.php

lib/public/Defaults.php

tests/Settings/Mailer/NewUserMailHelperTest.php

tests/data/emails/new-account-email-custom.html

tests/data/emails/new-account-email-single-button.html

tests/data/emails/new-account-email.html

tests/lib/Mail/EMailTemplateTest.php