Clean errors in CSS mark-up in base/style/blocks.css

Found while fixing MDL-48202. Minor glitches but need to be reported and fixed.
1. https://github.com/lazydaisy/moodle/blob/master/theme/base/style/blocks.css#L57
2. https://github.com/lazydaisy/moodle/blob/master/theme/base/style/blocks.css#L58
Where (1.) was a throwback from 1.9 and (2.) was defining margins where none were needed, but also contained an error in the mark-up.