Any WordPress Theme Builders Out There?

[techlists at phpcoderusa.com]

I,

I'm learning how to create a WordPress Theme.  I think I understand 
enough to build my first theme.  The problem is I don't know what I 
don't know.

I'm stuck on create the CSS.

I went through Gutenberg and made a list of the CSS classes WordPress 
uses.  So far so good.

This is my home page under an existing theme  :  
https://www.phpcoderusa.com/

Here is an except of the HTML:

- - -
article id="post-1717" class="post-1717 page type-page status-publish 
hentry">
	<header class="entry-header">
		<h1 class="entry-title">PHP Coder USA is a Full-Stack Linux and 
Full-Stack PHP Tutorial Website and YouTube Channel.</h1>	</header><!-- 
.entry-header -->


	<div class="entry-content">

<div class="wp-block-image"><figure class="alignleft size-full 
is-resized"><img 
src="https://www.phpcoderusa.com/wp-content/uploads/2018/06/front_page_Keith-Smith-PHP-Programmer.png" 
alt="Image of Keith Smith, PHP Programmer" class="wp-image-29" 
width="201" height="201" 
srcset="https://www.phpcoderusa.com/wp-content/uploads/2018/06/front_page_Keith-Smith-PHP-Programmer.png 
200w, 
https://www.phpcoderusa.com/wp-content/uploads/2018/06/front_page_Keith-Smith-PHP-Programmer-150x150.png 
150w" sizes="(max-width: 201px) 100vw, 201px" /></figure></div>
- - - -

This little bit of code has the following classes:

post-1717 (What is this number?)
page
type-page
status-publish
hentry
entry-header
entry-title
entry-content
wp-block-image
alignleft
size-full
is-resized
wp-image-29 (What is this number?)

I've looked at existing themes CSS and that is not as helpful as I would 
like.

I've searched for tutorials on how to style WordPress and I am not 
finding much. I'm surprised given it is reported that WordPress is used 
on 30% of the Internet websites.

Any suggestions on books, websites...etc?

Is there any WordPress groups I might join so I can get some help.  I 
think I am close.... so I think!!

Much thanks in advance!!

Keith