FitNesse. UserGuide.
MarkupStyle [add child]
To set the style of a word or phrase use the !style_(text) widget.
This widget has three forms. !style_(text), !style_{text}, and !style_[text]
Yes, that means that you cannot put a style around a single string that has all three closing terminators. I'm sure you can figure out the workaround.

Here are some of the available classes you can use. Remember you can (should) add more classes to: FitNesseRoot/files/fitnesse.css

Markup Text
Displayed as
!style_error(an error) an error
!style_pass(pass) pass
!style_fail(fail) fail
!style_ignore(ignore) ignore
!style_caps{Caps} Caps
!style_code[code] code
!style_red[red] and other colors... red, orange, yellow, green, blue, indigo, violet
  pink, lightYellow, lightGreen, lightBlue
  chocolate, darkOrange, darkRed, olive, darkBlue, darkGreen
!style_code(!style_red[code red]) code red
!style_note[note] note
!style_page_title[page_title] page_title
!style_right[right] right
!style_strike[strike] strike
!style_meta[meta] meta
!style_included[included] included
!style_setup[setup] setup
!style_collapse_rim[collapse_rim] collapse_rim
!style_pageHelp[pageHelp] pageHelp
x !style_hidden[hidden] x x x
!style_test_output_name[test_output_name] test_output_name
!style_fit_label[fit_label] fit_label
!style_fit_stacktrace[fit_stacktrace] fit_stacktrace
!style_fit_grey[fit_grey] fit_grey