All the CSS settings follow a "combine" approach.
In other words, if you enter
'green'
then you are setting the css to _just_ green.
If you enter
' green'
(ie space-green)
then you are _appending_ green to the classes for that item.
This allows you to _append_ or _override_ the default css for that item.
As Kevin noted, it's also a space separated list - so
' green bold'
applies the default, then green, then bold.
Cheers
Bruce