[stable15] Use SCSS variables since CSS vars are not loaded on public pages

[backportbot-nextcloud]

backport of #14670

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 16981: failure

TESTS=acceptance, TESTS-ACCEPTANCE=app-files

• cancelled - typically means that the tests took longer than the drone CI allows them to run

TESTS=acceptance, TESTS-ACCEPTANCE=apps

• tests/acceptance/features/apps.feature:75

Show full log

  Scenario: View app details for app store apps   # /drone/src/github.com/nextcloud/server/tests/acceptance/features/apps.feature:75
    Given I act as Jane                           # ActorContext::iActAs()
    And I am logged in as the admin               # LoginPageContext::iAmLoggedInAsTheAdmin()
    And I open the Apps management                # SettingsMenuContext::iOpenTheAppsManagement()
    And I open the "Tools" section                # AppNavigationContext::iOpenTheSection()
      Tools section item in App Navigation could not be found after 100 seconds (NoSuchElementException)
    When I click on the "Antivirus for files" app # AppsManagementContext::iClickOnTheApp()
    Then I see that the app details are shown     # AppsManagementContext::iSeeThatTheAppDetailsAreShown()