How to make a theme from scratch for a web app?

I have got a task to build the prototype of e-commerce like website from scratch. It needs to be done with Angular. But that is not important for me now. I don’t know how to make a theme for the application?

I have not made any theme before and not finding good article or tutorial on this sector. Can anyone suggest me resources for making a theme for a web application? I know basic of SASS. So, I think I can use that for this case. But I am at a sea.
How do I structure the theme?
How many color variables are needed to be declared?
How to customize if I use Bootstrap or others?

Thanks in advance.

If you know HTML/CSS and it is not important which service you are going to use you could check the following series:

Wordpress Theme Development by Traversy Media (please search this on Youtube as I am not allowed to post a link yet, it is a series of 10 videos)

You can build an e-commerce service using WordPress & its plugin WooCommerce.

The series is made by Brad Traversy and it is about how to make a WordPress theme from scratch based on Bootstrap.

Now there is a newer version of Bootstrap available, so you will notice some changes, but most of them are indicated in the videos’ comments.

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Thank you Giordanidis,
I am not actually using Wordpress, the task is needed to be done without any CMS. But I’ll check the tutorial for theme development learning. I wanted to know how to make a theme from scratch which can be used throughout the website/ web application. I hope the tutorial serve that purpose.