Add more board themes

[PartyPlanner64]

There is somewhat of a framework present for supporting "board themes" or, in other words, default assets that a board can start out with.

To add a theme, create a subfolder within the themes directory and include images that are named the same as the ones in the default theme folder. Then update newboard.jsx to recognize the theme.

I am not an artist so I will gladly accept reasonable themes to be included.