#primary-menu li a {
font-weight: 500 !important;
font-size: 15px !important;
}
The custom css that having greater than(>) symbol is causing this issue, after saving the changes the > symbol is getting converted into >
and thus becomes an invalid CSS.
Can you pls upload the childtheme and move all the custom CSS there.
(Now there is no such issue since added modified CSS but later on if you use custom CSS with > then it will be an issue.)