StyleBox[boxes, options] represents output in which boxes are shown with the specified option settings.
StyleBox[boxes, "style"] uses the option setting for the specified style in the current notebook.
You can use font options such as FontSize, FontWeight, FontSlant, FontFamily, FontColor and Background in StyleBox.
The following additional options can be given:
In StandardForm and InputForm input, StyleBox is by default ignored, so that StyleBox[box, spec] is interpreted just as box would be.
When StyleBox objects are nested, the options of the innermost one control the display of particular boxes.
In StandardForm, explicit StyleBox objects are output literally. You can use DisplayForm to see the display form of such objects.
See The Mathematica Book: Section 2.8.10.
See also: StyleForm, AdjustmentBox, FrameBox, Cell, ShowAutoStyles.