How to make a design for angular app?

Question

HTML

Friends, tell me. There is an application that was written by a third-party developer. When making money, he used a theme bought from themeforest . Now I am faced with the task of making my own design.
How to do it right? I take the bootstrap head and redefine its styles in order to get what's in the layout?
Or I write my own css, and in the code I use only certain necessary bootstrap classes