Theme Options > Layout - Different options for header and content

Kushal Lakhan You might be able to target the header by injecting some custom CSS into an Unfiltered HTML widget within your Blog's theme. For example, using this community:

Once you know the correct CSS class(es), add something like:

<style>
html body .element .element_1 {
    property: value;
    }
</style>

This should override any of your site's default CSS, but you may need to trial & error some changes to your code to make this work.

Are you trying to blow out the margins on a specific blog?  We've done it here with a custom less file in our theme, then we dynamically add it to the header via the "HeadScripts" area in the theme when the blog in question is loaded.

Here is a link to our blog that blows out the margins to have a 100% width for one zone on the layout.
https://www.hearingfirst.org/b/blog 

Are you trying to blow out the margins on a specific blog?  We've done it here with a custom less file in our theme, then we dynamically add it to the header via the "HeadScripts" area in the theme when the blog in question is loaded.

Here is a link to our blog that blows out the margins to have a 100% width for one zone on the layout.
https://www.hearingfirst.org/b/blog 

I want to apply this to any new blog posts created within certain groups. If the changes you mention apply to the theme within a singular blog post, the same theme should apply to any new posts created as well. 

At least, this is the behaviour I've observed when playing around with the settings.