An important theme, which is actually the theme that moves the plot forward, is negative influence. Lord Henry's main goal since he first laid eyes on Dorian Gray was to change Gray's life and to make it fit Lord Henry's own whims. Lord Henry was able to read into Dorian's moral weakness, and even went as far as to use Dorian's unhappy family history as fuel to set the perfect scenario of corruption that he had set out for the young man. He knew that it would work because he used every technique possible to get through straight to Dorian's psyche. Ultimately, it was this negative influence what destroyed Dorian's body and soul.
Another salient theme is class and social differentiation. The aristocratic background of Dorian makes him more likely to listen to an equal, like Henry, rather than to follow the righteous path to good living proposed by his former best friend, the poor painter Basil. It is also class differentiation that seems to give Lord Henry the feeling of entitlement to control the lives of those around him. Certainly, he transferred that same feeling onto Dorian who, instead of changing, basically destroyed everyone in his path.
A final theme that could be considered is one very dear to Oscar Wlde: art imitating life, and life imitating art. This theme is very salient in Dorian's obsession with Sybil Vane in Chapter 3. Notice how his primary attraction for Sybil is the fact that she is an actress who plays all of Shakespeare's heroines
She is all the great heroines of the world in one.
It is the idealized female that she represents when she acts, and not the real woman, that fuels Dorian admiration. In the case of art imitating life, Dorian intends the opposite; he wants (like Henry) for life to be a trivial exploration where art dominates one's every actions no matter how serious they are. Under an aesthetic, Wildean, perspective life and its daily torments is nowhere as important and interesting as the excesses that can only be possible when applying an artistic approach to it.
"To-night she is Imogen," [Dorian] answered, "and to-morrow night she will be Juliet."
[Henry:] "When is she Sibyl Vane?"
"I congratulate you."
Therefore, these themes help mold and move the plot of the novel by applying the main ideals of Wilde's style: life imitating art, the eternal search for sensations, the unfair reality of social imbalance, and bad influences.