What you need to change is the "target" attribute of the anchors. They are most probably set to target="_blank" now, which cause the link to open in a new, blank, page. What you instead need is target="_self" which is also the default, so removing all target attributes would do it. As far as I know there is no way to do this with css, you have to edit the html files. So if your link looks like this:
HTML Code:
<a href="http://www.google.com" target="_blank">Link</a>
Change it to this:
HTML Code:
<a href="http://www.google.com">Link</a>
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