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.