Hi,
This all depends on the version of Moodle you are using.
Years ago it was much easier to build a theme based on the ideas given in a project by a client. I made many themes for a company in Switzerland some years ago and it was just a matter of Trial and Error getting the look right for the client. Once you have a predesigned look to a website putting it all together is relatively simple if you understand how Moodle works, and that you are familiar with the different languages used in the process such as PHP, mustache, SCSS, etc., to name a few, then it becomes easy over time as you learn new tricks.
Having said all this you do need to use the outline of the Parent theme in this case Boost or Classic depending on your design for the layout you are after.
You must remember too that lots of the layout can be done using the various options in the TEXT EDITORS within Moodle itself. Thus allowing you to create some nice looking pages without needing to write lines of code in the background of the theme for these pages as the layout would be basic HTML.
There is a link to the Bootstrap 4 at the top pf this Forum page that I added some time ago, which hopefully will give you a few ideas.
Hope this helps<
Cheers
Mary
