What is a theme found both in Heart of Darkness and Tess of the d'Urbervilles? Need to be able to find specific examlpes in both stories.
One possible theme to explore in both of these works of fiction is the repercussions or consequences of leaving one's native land and environment for distant shores. In Heart of Darkness, we see this idea explored in both Kurtz and Marlow. In both cases, however, it is the destination that appears to "pollute" and "corrupt" their characters. In Tess of the d'Urbervilles, we see this idea explored in Angel Clare, who leaves Tess and travels to Brazil where he becomes nothing more than a "mere yellow skeleton." In Angel's case, it is not Brazil that seems to have the effect on him as much as it is leaving Tess and Wessex.
I'm sure others will weigh in with other potential themes as well.