Can I use themeforest theme in final backend projects?

I can either create full stack projects in my own css, html or I can use professional looking theme from themeforest. Now, After finishing FCC, I plan to use ready made templates for projects. As these themes are really big, I think I will have good experience in using these themes. Plus, My final projects will look really professional.

So, my question is can I use these themes in final projects? I hope it wont dilute my chances of getting back end certificate. If it is against the rules, I will drop the idea and try to build everything from scratch.