RFR: Css updates

[Andreas Lundblad]

* Added styling for code blocks and info boxes.
* Joined code blocks into a single one in hgHelp and tClone.

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Commit messages:
 - Joined code blocks into a single one in hgHelp and tClone.
 - Added styling for code blocks and info boxes.

Changes: https://git.openjdk.java.net/guide/pull/22/files
 Webrev: https://webrevs.openjdk.java.net/guide/22/webrev.00
  Stats: 50 lines in 3 files changed: 32 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/guide/pull/22.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/22/head:pull/22

PR: https://git.openjdk.java.net/guide/pull/22