You could set the app font to bold and then override the post font with the snippet I gave you. Example:
/* App font */
body, input, button, textarea, select {
font-size: 84%;
font-family: Trebuchet MS,verdana,geneva,helvetica,arial,sans-serif;
font-weight: bolder;
}
/* Post font */
div.message-text, div.root-text {
font-family: Trebuchet MS,verdana,geneva,helvetica,arial,sans-serif;
font-weight:normal
}
You should remember that your "Trebuchet MS" font will only work properly on computers that have this font installed. Computers without this font will use those alternative fonts listed in the row. So the configuration you want will force the bold on those other fonts and the text may not look as great as they would look with Trebuchet MS.