Frenzie has it right .... Your main index.css style sheet has the following:
strong {
	color: #ffa50a;
	font-weight: normal !important;
}So you are forcing strong (bold) text to be normal.
Not knowing why you did that, you could probably open up the _dark/index_dark.css file and change 
strong {
	font-weight: bold;
} to target just the bbc version of strong (bold) as
strong.bbc_strong {
	font-weight: bold !important;
}But again not knowing why you globally changed strong that may not be what you want.