Add style for blockquotes in Garland

Split from #197425: Usability: enable BLOCKQUOTE by default:

Blockquote elements are currently unstyled in Garland, and are recognized only by their indentation. The theme would give a much better first impression of Drupal if quotes had a nice layout. Several ideas were proposed and rejected in the above-linked issue, but in order to focus that issue on the default allowed tags in the Filtered HTML format, the Garland question should be dealt with in this separate issue.