Changing background and button colours on public facing pages (like contribution) in drupal
In Drupal you could add a new theme, for example 'Bootstrap', and add the module 'Themekey' to Drupal.  Then in Themekey settings you can specify the Bootstrap theme to be used for a particular page. For example in the Themekey settings when 'drupal.node' = 'civicrm/something' then theme is 'Bootstrap'.  No coding required!