If you view source in your browser, you will See that the titles markup is <h2 class”title”>This is a title</h2>.
( There’s some other stuff in there too but this is the CSS class for your titles )
You can give this class any style you like in your CSS file. The style rules are in styles.css in your nolimits theme.
You already have a style for #content h2, which means all heading 2 in the content div. Search for that in styles.css and you will find a little further down a style for
#content .title a
This is for styling the anchor ( or link if you will )
Now right after the #content h2 rule, you can add
#content .title{
font-weight: bold;
}
There are other ways to do this, but I find this the most straight forward, given the existing stylesheet.
You can find a good CSS tutorial at W3 Schools
Good luck!