The Documentation theme is designed so the "Powered by Atlassian" footer is always visible and thus very prominent. (The windows is separated in three elements, the header, the footer and the "page" (splitter) with the sidebar to the left and page content to the right. These three are always visible, at least on Chrome/FF).

Currently (5.5.2), the footer div's "footer-body" section has a min-height property with a value of 44px, meaning that the footer will take up an effective 55 pixels vertically on the screen. For many screens, especially laptops and mobile devices, this is far too much. Not only is it annoying for the user to have that much real estate dedicated to a non-informative footer. Worse is, it is aesthetically displeasing to look at.

The new default look moves the footer to the bottom of the page, so it is only visible when the page is scrolled all the way down. This way, it doesn't matter that the page has a huge, prominent footer. However, for the Documentation Theme, the always-visible footer should not be much larger than what is required to display the "Powered by Atlassian" line of text.

Removing the min-height css property of the footer-body section makes the footer adjust size automatically to display the "Powered by Atlassian" line of text with sufficient padding. The result is much more aesthetically pleasing than before.

To reproduce:
1) Open a page that uses the Documentation Theme (I assume Chrome in the following, because it has neat Developer tools).
2) Observe the excessive height of the footer, which has a lot of negative space below the "Powered by Atlassian" line.
3) Open up Chrome's "developer tools" panel.
4) Make sure the "Elements" tab is selected, and "Styles" are presented to the right.
5) In the elements display, open body -> div full-height-container -> div footer -> section footer-body. (or right-click the footer and select "show info on element".)
6) With section footer-body styles displayed, scroll down and find min-height : 44px. Disable this property.
7) Resize the window to force update, and observe how the footer looks much better.

Note: Confluence 5.5.3 (m2) adds a margin: 10px to the <ul> element in order to "even out" the excessive negative space below the "Powered by" line in 5.5.2. The footer is now less ugly, but still takes up much more real estate than needed to display the footer, which still makes the user think that Atlassians are major douchebags with their major need for showing off.

Note 2: Please check this issue on a screen with a vertical resolution of < 800 pixels to get the point.