Hi, @maldor
I have been developing a lot things with Joomla cms for more then 6 years and recently was asked to take a look at Prestashop and migrate current Joomla site to Prestashop or Magento. I have never ever used Prestashop and it took me like 1 month to find how it works.
Now regarding your question.
Who ever has created Classic, Classic-rocket themes has probably HTML and Bootstrap 4 knowledge of zero. There is so many html issues in those templates. Basically you need to go throught all tpl files and fix them. As example in Classic theme there is:
<button id="submit-login" class="btn btn-primary" data-link-action="sign-in" type="submit" class="form-control-submit">
{l s='Sign in' d='Shop.Theme.Actions'}
</button>
As you can see class attribute is declared twice for no reason... Seems prestashop is proud of using Bootstrap 4 but in classic theme there is like only 5% usage of it.
It has some kinda custom classes like "hidden-sm-down" which u don't need cause you can do same with default BS4 utilities "d-none d-md-block". Only using BS4 you can get rid about 70% of theme.css....
So what i am doing now. Created new theme based on classic theme, got rid of all css, js....Left only core things. Going throught all tpl's files and doing my customizations there.
So my answer is YES, if you want to have clean theme then you must do it from scratch. Probably it doesn't matter which default theme you gonna choose cause both of them are crap for production usage....
Thanks