I new theme would be great indeed
All Theme related files are in /usr/local/ispconfig/interface/web/themes/
Make a copy of the default folder and name it e.g. newtheme and then edit the files within this folder. The current theme is just a "developer" version based on old style tables and not div / css and should be replaced until the final version.
If you have any questions, either post them in the devlopment forum or send me a pm.