Themes
You can determine yourself which of the included themes you want to offer on your site. The theme files are located in the folder “styles”. You can remove theme files that you do not want to display to your users. Remove the theme file itself and the corresponding CSS file.
In the admin menu you can determine what will be the default theme. Admin page -> Control -> Settings. Use option “Default skin”.
Your visitors will see that theme by default on a first visit. They can then choose another theme if they want to, and their choice will be saved in a cookie for further use.
If you leave only one theme file and set that as the standard theme, the selection box will no longer be displayed.