The theories and concepts I have discussed on this website are not exhaustive and there are many ways of approaching graphic design projects or critical writing through theory. In fact, often the best projects are the ones that demonstrate original application of theory.
Here are some other theoretical concepts that may be worth exploring:
Eco-feminism
Post-colonial theory
The Culture Industry and the Frankfurt School
Psychoanalysis
Symbolic Interactionism
See what you can find out about these ideas but remember to evaluate your sources! It is OK to use general web sources (such as this) to get a gist of complex topics however, if you aim to use these in your work then you should try to engage with more complex literature written by experts.