Obviously there are lots of variations in dystopian literature, and so there is no one size fits all approach. However, there are some themes that can be gathered together that occur in many dystopian classics such as this novel and others. I will comment on the three main ones.
1) Dystopian literature, being set in the future, makes a comment on us today and often paints a bleak picture of the kind of future we will face. Normally dystopian literature is set in a distant future after some kind of nuclear holocaust or environmental disaster that has ended civilisation as we know it and greatly reduced mankind in terms of their level of technology. Thus is it that this story refers to the Ruin that has so dramatically changed civilisation.
2) Normally the new kind of government or power that has formed and has control over the people is one that is shown to abuse that power and to greatly restrict the rights of that people, pretending that such curtaling of rights is theoretically for the people's own good. Thus the Guardians, Kira discovers, are actually trying to manipulate and control their talents rather than giving them free expression.
3) The protagonist(s) of dystopian novels normally find themselves in conflict with the leadership or government and this external conflict normally shapes the plot of the novel. We can see this in Gathering Blue by the increasing sense of unease that Kira has with her discoveries about the Guardians, culminating in her decision when her father re-appears.