The new theme has been brought in to provide the following benefits: -
Faster single click navigation to sub-menu options
Allow all menu options can be seen, making it easier to work out which page is needed for a particular task
Up to date aesthetic
The new font is slightly shorter but also slightly wider than the font used in the old theme. We have had several comments about the font size, and will look into tweaking this to make our page more readable.
However, it is inefficient to develop code for both themes simultaneously, so I am afraid that the old theme will no longer be available in a few weeks.