property ConditionalFormat.Font as IFontDisp
Retrieves or sets the font for objects that match the criteria.

TypeDescription
IFontDisp A Font object that's applied to items or columns.
Use the Font property to change the font for items or columns that match the criteria. Use the Font property only, if you need to change to a different font. 

You can change directly the font attributes, like follows:

The following VB sample changes the font for ALL cells in the first column:

With Group1.ConditionalFormats.Add("1")
    .ApplyTo = 0
    Set .Font = New StdFont
    With .Font
        .Name = "Comic Sans MS"
    End With
End With