I read through the bug just now, and while the title just says 'styles', the bug covers both styles and themes. The bug is marked 'unassigned', which indicates that none of the developers have taken it up yet, so it is not yet resolved.

Styles and themes are a little different. As I understand it (and someone with more experience, feel free to correct me), styles are the basic structures for the way everything appears and is arranged in the journal. Themes are variations starting from that style. Styles say what is able to appear (header, footer, sidebars, entries, icons, etc.) and their basic properties (where on the page, how big, etc.). Typically themes will alter the colors; sometimes themes will alter the placement of journal elements like sidebars, or decide whether to show or hide something.