New content type - does not display paragraphs correctly

You have to override the behavior of droopler_theme_preprocess_page(), by adding the class "container-fluid" to $variables['main_attributes']

I added:

function step_subtheme_preprocess_page(&$variables) {
$variables['main_attributes'] ='container-fluid';
}

unfortunately it still doesn't work properly.
Have I done something wrong?

$variables['main_attributes'] is an object of the class Attribute, so you can't assign a string to it

I have a new type of content: gallery
Unfortunately, I can't define the function correctly.
Can you tell me exactly what it should look like?
I will be very grateful.