Hi Neil
At the top of the page, you can overlap by using the Header Overlap Page Setting:
Page Settings
You can adjust the transparency on scroll by adding the following to Custom CSS:
.site-header.stuck {
background: rgba(255,255,255,0.5);
}
255, 255, 255 can be changed to your color of choice. 0.5 can be changed to your chosen opacity. If you want to also adjust the header bottom border on scroll you can rather use:
.site-header.stuck {
background: rgba(255,255,255,0.5);
box-shadow: 0 1px 1px rgb(0 0 0 / 8%);
}