Hi Akshay,
I began developing a custom theme earlier this year. I think the best place to start, as Preben mentioned, is to create a child theme from Boost.
At the beginning you can just add a CSS file in your theme's config.php and start there.
For modifying the nav drawer, there may be plugins that will let you add fields, etc. but if you want to totally modify it, you need to learn what your theme's layout.php file is doing as well as what is up with the .mustache files in the templates folder, etc.
This approach will take you time, but you can pick up PHP and learn the framework if you keep at it. Good luck!
I began developing a custom theme earlier this year. I think the best place to start, as Preben mentioned, is to create a child theme from Boost.
At the beginning you can just add a CSS file in your theme's config.php and start there.
For modifying the nav drawer, there may be plugins that will let you add fields, etc. but if you want to totally modify it, you need to learn what your theme's layout.php file is doing as well as what is up with the .mustache files in the templates folder, etc.
This approach will take you time, but you can pick up PHP and learn the framework if you keep at it. Good luck!