[PRODDEV-184] fixed a size of buttons on the start screen of AF4 for the mobile screen for the Carnation theme

[aleevas]

Original Issue, this PR is going to fix: https://openy.atlassian.net/browse/PRODDEV-184

This should fix broken Date Time button on start screen of AF4 for the mobile screen for the Carnation theme I've decreased a little bit font-size of buttons for mobile screens:

Steps for review

• Go to the AF4 start page
• Check buttons on mobiel screens (tablet, mobile)
• Verify that it looks good

General checks

• All coding styles are fulfilled and there are no any issues reported by CodeSniffer. See Code of Conduct.
• Documentation has been updated according to PR changes.
• Steps for review have been provided according to PR changes.
  
• Make sure you've provided all necessary hook_update_N to support upgrade path.
• Make sure your git email is associated with account on drupal.org, otherwise you won't get commits there.
  
• If you would like to get credits on drupal.org, check documentation.

Thank you for your contribution!

[gundevel]

Can one of the admins verify this patch? Use "o+k to test" or ''t+est this please" for manual build execution.