As Quoth indicated font support in MOBI7 format is extremely limited. I suggest using bold, italic, and underline to distinguish different content in your case.
The Kindle Formatting Guidelines has a section on
Media Queries. You can target MOBI7 format using "@media amzn-mobi" and KF8 (and newer) using "@media amzn-kf8".
Note that MOBI7 does not truly support media queries. They are processed by kindlegen as part of the creation of that format from the source document.
(Also, the
Kindle Formats subforum is a better place to ask this sort of question in the future.)