Changes between the orgin and the firecups theme:
Copy the template, the assets folder and the ispconfig_version file into a seperate folder under /var/ispconfig/theme/
Switch the theme in the user settings.
Open the file templates/main.tpl and change the <html lang="en">
agains <html lang='en' data-theme="COLOR-OPTION">
You can use dark, blue, red, green as option. Without changes, the theme use the default color scheme.