In that case you will need a macro and a bookmark. Bookmark the text you want to highlight and call the Bookmark (say) BM1. Assuming the title of the checkbox control is Check1, the following macro goes in the ThisDocument module of the document. The macro files after you click outside the check box having checked or unchecked it.
You can add more case statements for other checkbox/bookmark combinations as required.
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Select Case ContentControl.Title
Case Is = "Check1"
If ContentControl.Checked = True Then
ActiveDocument.Bookmarks("BM1").Range.HighlightColorIndex = wdYellow
Else
ActiveDocument.Bookmarks("BM1").Range.HighlightColorIndex = wdNoHighlight
End If
End Select
End Sub