The WordPress admin theme has been overhauled for the next version (scheduled to be released mid-March), which means that a lot of plugins’ admin pages could end up looking out of place.
Joost de Valk gives some brief tips on how to mark up plugin admin pages to take advantage of the new styling.
Unfortunately, as he points out, the development admin style is using non-semantic markup, such as a class name of “niceblue.” I’ve created a patch, so hopefully this will not still be an issue when 2.5 is released.
Update: “niceblue” has been changed to “form-table”, thankfully.