Not actually seeing issue A – but theres a problem here.
Your header is a different height to the sticky header – so the offset needs to be different but the off canvas was never designed for this so theres no way to adjust for that.
So first off in @media (max-width: 768px) {
media query there is this CSS Rule:
#generate-slideout-menu.offside {
left: -100vw;
top: 100px;
}
Adjust the top: 100px;
value so it looks correct in A.
Then in the same media query you have this rule:
.offside-js--is-open .slideout-overlay {
display: none;
}
Which is hiding the overlay, remove that. And add this in its place:
.offside-js--is-open .slideout-overlay {
z-index: 1000;
background-color: #fff;
pointer-events: none;
}
That will simply make the overlay white so theres not noticeable gap on scroll.
Off Canvas menu items typography can be set in Customizer > Typography – its in the list of Target Elements.
If you need any custom styles outside of those, then can you raise a new topic for that.