There is not much to configure yet. You can enable and disable the toolbars and the statusbar in the "Settings" menu if you run out of space on your screen. Other options are configured in the Settings->Settings dialog.
There exist three different themeable data sets:
names theme: property names and money factor, e.g. localized versions for different countries
icons theme: icon sets displayed on fields, e.g. free parking, jail, ...
dice theme: pixmaps of the rolling dice