Diary ships with the two styles you know from Data Center. Pick one with the Diary style parameter - see Macro Settings.
Simple
The default. Each entry is a compact block, ruled off from the next by a horizontal line. The header carries the date, the time (if Display Time In Entries is on), and the author's name.
Bubble
Each entry sits in its own rounded, raised box beside the author's avatar, with the author's name leading the header rather than trailing it.
Switching between them
Style is a macro setting, not a per-entry setting: every entry in one Diary uses the same style. Changing it does not touch the entries themselves, so you can switch back and forth as often as you like without any risk to your content.