Use a regex:
public static Regex regex = new Regex(
"(?<=FONT-FAMILY:\\s)(?<Font>'.+')(?=;)",
RegexOptions.IgnoreCase
| RegexOptions.Multiline
| RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
public static string regexReplace = "${Font} !mportant";
string result = regex.Replace(InputText,regexReplace);
Revised version:
public static Regex regex = new Regex(
"(?<=FONT-FAMILY:\\s)(?<Font>'.+?')(?=;)",
RegexOptions.IgnoreCase
| RegexOptions.Multiline
| RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
public static string regexReplace = "${Font} !mportant";
string result = regex.Replace(InputText,regexReplace);