Is it possible to change the design? Which files have to be modified
(Sorry my english is bad)
If you want, you can change everything at the web interface. But keep in mind that you need more time to install an update of the JS3ServerMod Hosting Edition.
For less experienced PHP programmers I recommend only to create a new css file in the data/css/ directory of the web interface. All css files in that directory will be displayed in the web interface settings, so you can switch to it. This way you can update the web interface very easy, because you did not overwrite any official web interface files.