There are many disciplines in science that a student can draw a theme for a thesis from. The best way to find a theme is to pick a subject, like genetics for example, and look at the recent research that has been done in that subject. If you see a gap in the research or can formulate an idea that the researchers did not think of, you can use that idea for a theme. Sometimes you can take the results of two or three research papers to see if they dispute each other or if there is error in validity or reliability in their processes.
If you are passionate about a person or subject in history, finding a thesis theme in history is not as difficult as it would seem. For example, you are passionate about George Washington. There is a lot of resources with information on the first president. Research and read as much as you can. Keep careful notes. In all the words that have been written about Washington, all information is not covered. What was the name of his staff? Is there a specific staff member where there is just a little information? Really dig your heals into that niche of history nobody has exploited.
If you are studying business you can gain ideas for a theme thesis by researching what you are interested in. You can find themes in economics, management, leadership or just organizational structure. Though each of these categories are broad, a little research of recently published peer-reviewed articles or literature reviews can give you an idea of recent research and where you might go with your own research. Globalization of business is a hot topic that hasn't been fully researched because the field and the study of a global economy is presenting new challenges every day.
Literature is a very broad field that offers endless ideas for themes. If you are looking for a thesis theme, remember that the older the work, the more has been written about it. You might want to research recent literature that has been written in the last hundred years. You will find gaps in research and possibly find a niche where you can exploit your thoughts and research. Literature is interpreted in a myriad of different ways and your interpretation of a piece of work is usually more flexible to peers than other fields. Literature reviews in peer-reviewed journals are excellent resources to get new ideas.