Improve CSS for block tags

There are some issues with how block tags are currently presented in the browser:

 - Block tag content is not indented, making it difficult to distinguish from the main description.
 - Lists within block tag content use the styles for nested lists and therefore have no margins.

These issues were identified as part of JDK-8330714. Since more thorough structural changes are being considered for JDK-8330714, this issue is created for these relatively simple CSS changes.