Base theme for control
This is a theme class option! Theme class options take a letter argument (jqm:a,b,c...), or a context class (bootstrap:danger,success,info...). However, this is really just part of a class name, so by adding a space, you can also include any arbitraty class name you want with the option. For more information, please take a look at the Theming Datebox section.
This option may be unused in bootstrap.