The background-image used for the Table of Contents sidebar in the GeoServer doc style is a 800px high image.
When the table of contents is less than 800px high, the background image is truncated, and displays normally.
When the table of contents is larger than 800px high, there is a gap between this background image and the top of the table of contents.
There are a couple routes to fixing this:
a) Add a tileable segment, to be inserted when the TOC is larger than 800 px.
b) Instead of using a background image, we can just use a CSS3 style with a linear-gradient fill and a rounded border. This will reduce browser compatibility a little bit, but all modern browsers support this functionality.