Please see for example the starting line in the core boxton template. It starts with a wrapper div with style <div class = "layout-boxton ... >
Each template can have such a wrapper HTML element with class with a unique name of this template name and through this unique class in CSS files of an active theme with different CSS selectors you can manage all active templates on the site, it is not necessary to have CSS code in the templates themselves.
And to note something you may not have noticed yet - in Drupal, when you change the theme, all blocks are shifted or disappear if the new theme has different regions. Separating the theme from the layout in Backdrop CMS makes it so that changing the theme does not shift anything in the structure of the blocks, because they are located in the template, not in the theme. This is a huge advantage. That is why there is no CSS in templates - they are not responsible for the visual presentation of the site, but only for its structure.
And this is a significant step in achieving a very important and fundamental principle in web design - distinguishing content from presentation.
Another important advantage you will see as you become more familiar with the template system is that the templates and layouts based on them have a powerful logic controlled easy by the admin panel that makes them behave differently according to different contexts - something that was impossible with the good old page.tpl.php in Drupal.
I understand that it requires a bit of a mind shift to use layouts in BackdropCMS after creating custom themes in Drupal 7. However, I don't think it's anywhere near "ridiculous" once you wrap your head around it.
Having worked with layouts in Backdrop CMS for a long time now, I don't understand why you would think that custom layouts are needed for a new theme? I suspect that if you discused this with an experieced Backdrop CMS themer in a conversation, they might be able to provide the insight to help get you past this conception.
I don't know if this video I made might help you or not:
If you watch it, let me know if it was helpful.
We have our next Backdrop LIVE happening later this month and we'll have a live session on Layouts. This would be a great chance to ask questions and hear from experienced developers about how they use Layouts.
Please, keep using the forum to ask specific questions about layouts. You might never learn to love layouts, but I think you will at least be able to understand why layouts are useful and how to work without the frustration you seem to be experiencing.