Custom shading to highlight substrings

The section of the book on SQL injection needs to illustrate how an attack string gets inserted into a text string representing a SQL statement with unexpected results. The standard way of highlighting a part of a string available was to use bold like this:

INSERT INTO Grades (name, grade) VALUES ('Robert', ‘A+');–’, ‘F’);

However, it’s difficult to clearly distinguish a bold single quote, for example, because it’s so thin.

Edit: Again, the No Starch folks were eager to find a solution. I won’t attempt to match the font but here’s how it appears in proof pages:

example of shading in code excerpt

Now it’s crystal clear.