Google domains provides a really easy and intuitive method for linking new domains to Blogger. I have yet to find a decent guide on how to set up a new theme on Blogger (formerly Blogspot) so I've decided to write one myself based on my experiences setting up this page. I figured most of this out from snippets, trial and error and disecting a few freely available themes.
The bare minimum for creating a blogger theme is listed below, this will create a no frills empty layout for you to begin with, If you're using the below code as a base I recommend adding a few other sections alongside main to make your code easier to manage.
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE html>
<html xmlns:b="http://www.google.com/2005/gml/b" xmlns:data="http://www.google.com/2005/gml/data" xmlns:expr="http://www.google.com/2005/gml/expr" xmlns="http://www.w3.org/1999/xhtml">
<head>
<b:skin>
<![CDATA[
/* Styles go here */
]]>
</b:skin>
</head>
<body>
<div class="content">
<b:section class='header' id='header' showaddelement='yes' />
/* Header */
</b:section>
<b:section class='content' id='main' showaddelement='yes' />
/* Main Content */
</b:section>
<b:section class='footer' id='footer' showaddelement='yes' />
/* Footer */
</b:section>
</div>
</body>
</html>
Declaration of the three XML namespaces in the HTML tag is crucial as it will allow Blogger to interpret you can call these namespaces anything you like but from what I've seen available, most themes have landed on the above naming convertion. The "b" namespace is the one will be using most commonly thoughout the tutorial.
Inside our <head> tags we have our first blogger namespace element <b:skin> this is our container for the CSS on the page, use a CDATA tag around your CSS to make sure you don't encounter any character encoding issues.
Inside our <body> tags we have our first <b:section>. It isn't strictly necessary to have 3 sections when setting up your initial page but will help us down the road so I recommend you keep them for now. Save the above text into to a .xml file then we're ready to load.
Open up Blogger, go to the Theme option. Hit the overflow menu on your current theme and select Restore
If the syntax is correct (and if you copied correctly from above, it should be) the theme will be loaded. Click on the layout tab and we'll see the three sections mentioned in the above code. Header, Main, and Footer.
That's a solid starting point, in the next section we'll look at the <b:section> tag and how to fill it with data.