Newbie - need to mimic my bs4 site theme... have basic questions

[ScubaPongo]

First time user with PS.  I have installed on a subdomain store.example.com and would like to mimic the design of my main site.  I am am very experienced with css, html, php, js, jquery and so on - ~15 years.  I use BS4 as my base usually these days as it goes by quicker and this is what I used for this particular main site.

Now, I need to create/edit a theme to mimic my main site.  The docs say to use the startertheme (https://github.com/PrestaShop/StarterTheme), but it looks like it hasn't been updated in forever.

I guess my main question is what is the starting point here to create a new theme or do I just copy the 'classic' theme and edit it or what?  I am used to doing sites with the actual code (no visual editor stuff) and don't use cms type stuff, but given some sort of direction on where to get started I am sure with some time I will quickly pick up on everything.

I want to use the latest BS 4.3.1 then can go from there making it the same/similar as to my main design.  Is there a rough 'guide' other than the PS docs to browse over somewhere or any advice anyone can give?