Finding prominent themes in a topic does not mean that you need to read everything that was ever written. Usually looking at the first handful of pages on databases can give you an idea of what themes within a topic have the most written about them. You can also use tools like the Subject: Thesaurus Term on any EBSCO database to see what the most prominent subjects are within the articles that have pulled up with your search.
When performing research on your topic, your goal is to find a few themes you can identify that seem to crop up a lot in your searches. You can then organize them into a cloud like the one above. The cloud represents you overall topic and each section within, separated by the dotted lines, is each theme you can find within the topic.
For example, if my topic was representation in movies, that would be my entire cloud. Now, when I start doing research I will find different themes like representation of race, sexuality, mental health, trauma, etc. These would become the sections within my cloud.
Once you feel like you have a good idea of what the biggest themes are, now you can turn them into buckets. The bucket metaphor helps because it symbolizes how we need to fill each theme with authors who have written scholarly articles within that theme. It's best to have more authors than themes because the idea is that if a certain theme is particularly popular in a given topic, then more people will have written about it.
The bucket metaphor also helps us organize our literature review. Like I said on the previous page, you don't want your lit review to just say "this person wrote this and this other person wrote this." You want to identify major themes and then when talking about those themes, you can cite the authors who have written about them.