CSS changes not taking effect

Hi
I’m completely new to website development and it’s a steep learning curve so go easy on me.
I’ve made a child of the twentytwenty theme and have attempted to make a number of formatting changes using both the ‘Customize > Additional CSS’ route and directly in the style.css sheet but none of the changes seem to show. This was the same before I made the child theme and added CSS to the parent twentytwenty theme also (or any theme for that matter).
The immediate problem i’m trying to solve is to remove the padding from the header, but in attempting to get any changes visible i’ve tried changing font and background colour etc all to no effect.
Based on my googling i’ve tried:
1) Clearing the cache – both manually and installing the Busted! plugin
2) Speaking to the webhost to clear the cache their end
3) Switching browsers
4) Using various code snippets in case the syntax was wrong
5) Making the changes through inspect element and adding that CSS to the style sheet.

.header {
    padding-top: 0px;
}

But like i said, at this point i’m just trying to get any style change to work. Thanks