Calm down @jefferis, two posts and both mention “restrictive”! There aren’t any restrictions, it’s just understanding how to apply the changes.
To change any of the predefined blocks background colour, you can use the dropdown menu block options, then change the relevant colour via the CSS class.
You can also add your own CSS class, such as “.restrictive-blue” and apply that to any element.
You can also change the relevant #header-1 .main-nav
You could create a new rule (attach to a new stylesheet for better management of your custom styles) for #header-1 .main-nav
Personally I prefer to create my own class and attach to a new stylesheet and I prefer not to use important (unless I have too).