Adding opacity to header navigation

Site URL: https://www.shineyouth.org.uk/

I would really welcome your help.  I have been asked to ensure the image on Shine Youth's homepage does not cut off the young people's faces.  It has been created with the 7.0 Bedford template. 

In order to achieve this I was looking to undertaking two things: 

1.  Add opacity to the header navigation (1st & 2nd images) - In the Bedford template you are only able to have full opacity or none.  I need to be able to edit the CSS so that you can see the image below and still read the header navigation. 

2.  Use more of the space above their heads of the logo isn't sitting on the boy's eye (2nd image) - The uploaded image has there is sky above their heads. (3rd image)

Despite familarlising myself with Squarespace, I have gone round in circles for most of this afternoon.  It would be great to resolve this issue before I leave due to Covid-19 reasons.

Many thanks in advance.

Add to Home > Design > Custom CSS

body.homepage header#header {
    position: absolute;
    background: rgba(0,0,0,0.5);
}
body.homepage .banner-thumbnail-wrapper.has-description {
    padding-top: 100px;
}