Eric's answer will work in the current version, but I don't know if that is always going to be supported. If you want a CSS option, you can use one of the following.

/* top and bottom text */
div > .cf-helptext {
  font-weight:bold;
}

/* top text */
div > .ab-help {
  font-weight:bold;
}

/* bottom text */
div > .un-help {
  font-weight:bold;
}

The selector should be specific enough to override the built-in styling, and depending on which you use will target the both text above and below, or just one of them, for all fields on the form (if it doesn't override it, try adding !important).

If you want to target specific fields/section headers, either look for the q# value for that field/section in the CSS/JavaScript page and add it to the front of the selector, or add a custom class like "boldHelpText" to your target fields and use that instead.

For example,

/* targets only the bottom text for the q16 field */
#q16 div > .un-help {
  font-weight:bold;
}

/* targets the bottom text for fields with the boldHelpText custom css class */
.boldHelpText div > .un-help {
  font-weight:bold;
}