The answer to this question rather depends on what themes you have already identified. However, one of the more interesting themes from my perspective is that of social change. This theme is based on the setting of the novel and in particular the way that WWI impacts on the characters. Let us remember it is this that has destroyed the life of Septimus. Also, the content of the novel makes it clear that we are entering a new world where social class has an ever-diminishing impact on a person's life chances. Consider the way that Bradshaw has now managed to gain a title even though he came from humble and obscure origins. Likewise, we are told that Septimus, before his wartime experience, was heading up the social ladder.
Other details that are refered to that point towards social change are the rise in power of the Labour Party and colonial unrest in India. The Labour Party in Britain traditionally has always represented the majority rather than the influential moneyed minority, and thus points towards a decline in the power of the aristocracy. The unrest in India also indicates that Britain's power is on the wane.
Consider to the presentation of gender through the character of Elizabeth Dalloway. Women like her are now able to have their own career, whereas before the only role open to women was to be a wife. Therefore, throughout this exciting and vibrant novel, social change, through focusing on formerly oppressed groups of society and the way that they now have options and rights, is a major theme.