Thanks, I'm able to reproduce it in my local env.
React is dropping some of the css rules meant to add gradient to header, it would be fixed in upcoming version.
For now you can add this css rule at bottom of styles.css to fix it for steelblue theme:
.pq-header-outer .pq-cont-inner {
background-image: linear-gradient( #fefefe, #dae6f0) !important;
}