Well, looking at the title 'Dating with the Dark Portal', a major theme could be the contrast between light and dark. The 'dark portal' represents the unknown, the dangerous, and perhaps the forbidden. And the 'dating' part might imply that there is an attempt to form a connection or relationship with this dark element. This could lead to themes of courage, as the characters are brave enough to engage with the dark portal. Also, it might touch on themes of discovery, as they uncover the secrets behind the portal. And there could be a theme of self - discovery too, as they learn more about themselves through their interactions with the portal.
One possible theme could be exploration. Since there's a 'dark portal', the characters might be exploring the unknown which often is a big theme in novels. Another could be transformation. Their experiences with the portal could change them. And of course, there could be a relationship theme, given the 'dating' in the title.